Creating an advanced-level Online Auction System involves designing a robust database structure, implementing a Flask backend, and creating a responsive frontend using Bootstrap 5. Below is a detailed breakdown of the project:

Database Models

from flask_sqlalchemy import SQLAlchemy
from datetime import datetime
db = SQLAlchemy()
class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(80), unique=True, nullable=False)
    email = db.Column(db.String(120), unique=True, nullable=False)
    password_hash = db.Column(db.String(128), nullable=False)
    role = db.Column(db.Enum('admin', 'user'), nullable=False, default='user')
    created_at = db.Column(db.DateTime, default=datetime.utcnow)
    items = db.relationship('Item', backref='seller', lazy=True)
    bids = db.relationship('Bid', backref='bidder', lazy=True)
    transactions = db.relationship('Transaction', backref='user', lazy=True)
class Item(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(200), nullable=False)
    description = db.Column(db.Text, nullable=False)
    starting_price = db.Column(db.Float, nullable=False)
    current_price = db.Column(db.Float, nullable=False)
    image_url = db.Column(db.String(500), nullable=True)
    seller_id = db.Column(db.Integer, db.ForeignKey('user.id'), nullable=False)
    start_time = db.Column(db.DateTime, default=datetime.utcnow)
    end_time = db.Column(db.DateTime, nullable=False)
    status = db.Column(db.Enum('active', 'closed'), nullable=False, default='active')
    bids = db.relationship('Bid', backref='item', lazy=True)
class Bid(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    amount = db.Column(db.Float, nullable=False)
    bid_time = db.Column(db.DateTime, default=datetime.utcnow)
    bidder_id = db.Column(db.Integer, db.ForeignKey('user.id'), nullable=False)
    item_id = db.Column(db.Integer, db.ForeignKey('item.id'), nullable=False)
class Transaction(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    user_id = db.Column(db.Integer, db.ForeignKey('user.id'), nullable=False)
    item_id = db.Column(db.Integer, db.ForeignKey('item.id'), nullable=False)
    amount = db.Column(db.Float, nullable=False)
    transaction_time = db.Column(db.DateTime, default=datetime.utcnow)

Flask Application Structure

online_auction_system/
│
├── app/
│   ├── __init__.py
│   ├── models.py
│   ├── routes/
│   │   ├── auth.py
│   │   ├── items.py
│   │   ├── bids.py
│   │   ├── admin.py
│   │   └── dashboard.py
│   ├── templates/
│   │   ├── base.html
│   │   ├── auth/
│   │   │   ├── login.html
│   │   │   └── register.html
│   │   ├── items/
│   │   │   ├── add.html
│   │   │   ├── list.html
│   │   │   └── detail.html
│   │   ├── bids/
│   │   │   ├── place_bid.html
│   │   │   └── bid_history.html
│   │   ├── admin/
│   │   │   ├── add_user.html
│   │   │   └── manage_users.html
│   │   └── dashboard.html
│   ├── static/
│   │   ├── css/
│   │   │   └── styles.css
│   │   ├── js/
│   │   │   └── scripts.js
│   │   └── images/
│   └── config.py
│
├── requirements.txt
├── run.py
└── README.md

Flask Application Code

app/__init__.py

from flask import Flask
from flask_sqlalchemy import SQLAlchemy
from flask_login import LoginManager
db = SQLAlchemy()
login_manager = LoginManager()
def create_app():
    app = Flask(__name__)
    app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///auction.db'
    app.config['SECRET_KEY'] = 'your_secret_key'
    db.init_app(app)
    login_manager.init_app(app)
    from app.routes.auth import auth as auth_blueprint
    from app.routes.items import items as items_blueprint
    from app.routes.bids import bids as bids_blueprint
    from app.routes.admin import admin as admin_blueprint
    from app.routes.dashboard import dashboard as dashboard_blueprint
    app.register_blueprint(auth_blueprint)
    app.register_blueprint(items_blueprint)
    app.register_blueprint(bids_blueprint)
    app.register_blueprint(admin_blueprint)
    app.register_blueprint(dashboard_blueprint)
    return app

app/routes/auth.py

from flask import Blueprint, render_template, redirect, url_for, flash
from flask_login import login_user, logout_user, login_required
from app.models import User
from app import db
auth = Blueprint('auth', __name__)
@auth.route('/login', methods=['GET', 'POST'])
def login():
    if request.method == 'POST':
        # Logic for user login
        pass
    return render_template('auth/login.html')
@auth.route('/register', methods=['GET', 'POST'])
def register():
    if request.method == 'POST':
        # Logic for user registration
        pass
    return render_template('auth/register.html')
@auth.route('/logout')
@login_required
def logout():
    logout_user()
    return redirect(url_for('auth.login'))

app/routes/items.py

from flask import Blueprint, render_template, request, redirect, url_for
from app.models import Item
from app import db
items = Blueprint('items', __name__)
@items.route('/items/add', methods=['GET', 'POST'])
@login_required
def add_item():
    if request.method == 'POST':
        # Logic for adding an item
        pass
    return render_template('items/add.html')
@items.route('/items')
def list_items():
    items = Item.query.all()
    return render_template('items/list.html', items=items)
@items.route('/items/<int:item_id>')
def item_detail(item_id):
    item = Item.query.get_or_404(item_id)
    return render_template('items/detail.html', item=item)

app/routes/bids.py

from flask import Blueprint, render_template, request, redirect, url_for
from app.models import Bid, Item
from app import db
bids = Blueprint('bids', __name__)
@bids.route('/bids/place/<int:item_id>', methods=['GET', 'POST'])
@login_required
def place_bid(item_id):
    item = Item.query.get_or_404(item_id)
    if request.method == 'POST':
        # Logic for placing a bid
        pass
    return render_template('bids/place_bid.html', item=item)
@bids.route('/bids/history')
@login_required
def bid_history():
    # Logic to show user's bid history
    return render_template('bids/bid_history.html')

app/routes/admin.py

from flask import Blueprint, render_template, request, redirect, url_for
from app.models import User
from app import db
admin = Blueprint('admin', __name__)
@admin.route('/admin/users')
@login_required
def manage_users():
    users = User.query.all()
    return render_template('admin/manage_users.html', users=users)
@admin.route('/admin/users/add', methods=['GET', 'POST'])
@login_required
def add_user():
    if request.method == 'POST':
        # Logic for adding a new user
        pass
    return render_template('admin/add_user.html')

app/routes/dashboard.py

from flask import Blueprint, render_template
from flask_login import login_required
dashboard = Blueprint('dashboard', __name__)
@dashboard.route('/dashboard')
@login_required
def dashboard_view():
    return render_template('dashboard.html')

Frontend with Bootstrap 5

app/templates/base.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <link href="https://cdn.jsdelivr.net/npm/bootstrap@5.1.3/dist/css/bootstrap.min.css" rel="stylesheet">
    <link rel="stylesheet" href="{{ url_for('static', filename='css/styles.css') }}">
    <title>{% block title %}Online Auction System{% endblock %}</title>
</head>
<body>    
    <nav class="navbar navbar-expand-lg navbar-light bg-light">
        <div class="container-fluid">
            <a class="navbar-brand" href="{{ url_for('dashboard.dashboard_view') }}">Auction System</a>
            <button class="navbar-toggler" type="button" data-bs-toggle="collapse" data-bs-target="#navbarNav" aria-controls="navbarNav" aria-expanded="false" aria-label="Toggle navigation">
                <span class="navbar-toggler-icon"></span>
            </button>
            <div class="collapse navbar-collapse" id="navbarNav">
                <ul class="navbar-nav">
                    <li class="nav-item">
                        <a class="nav-link" href="{{ url_for('items.list_items') }}">Items</a>
                    </li>
                    <li class="nav-item">
                        <a class="nav-link" href="{{ url_for('bids.bid_history') }}">My Bids</a>
                    </li>
                    {% if current_user.is_authenticated %}
                    <li class="nav-item">
                        <a class="nav-link" href="{{ url_for('auth.logout') }}">Logout</a>
                    </li>
                    {% else %}
                    <li class="nav-item">
                        <a class="nav-link" href="{{ url_for('auth.login') }}">Login</a>
                    </li>
                    <li class="nav-item">
                        <a class="nav-link" href="{{ url_for('auth.register') }}">Register</a>
                    </li>
                    {% endif %}
                </ul>
            </div>
        </div>
    </nav>
    <div class="container mt-4">
        {% with messages = get_flashed_messages() %}
        {% if messages %}
        <div class="alert alert-info">
            {% for message in messages %}
            <p>{{ message }}</p>
            {% endfor %}
        </div>
        {% endif %}
        {% endwith %}
        {% block content %}{% endblock %}
    </div>
    <script src="https://cdn.jsdelivr.net/npm/bootstrap@5.1.3/dist/js/bootstrap.bundle.min.js"></script>
</body>
</html>

app/templates/auth/login.html

{% extends 'base.html' %}
{% block title %}Login{% endblock %}
{% block content %}
<h2>Login</h2>
<form method="POST">
    <div class="mb-3">
        <label for="username" class="form-label">Username</label>
        <input type="text" class="form-control" id="username" name="username" required>
    </div>
    <div class="mb-3">
        <label for="password" class="form-label">Password</label>
        <input type="password" class="form-control" id="password" name="password" required>
    </div>
    <button type="submit" class="btn btn-primary">Login</button>
</form>
{% endblock %}

app/templates/auth/register.html

{% extends 'base.html' %}
{% block title %}Register{% endblock %}
{% block content %}
<h2>Register</h2>
<form method="POST">
    <div class="mb-3">
        <label for="username" class="form-label">Username</label>
        <input type="text" class="form-control" id="username" name="username" required>
    </div>
    <div class="mb-3">
        <label for="email" class="form-label">Email</label>
        <input type="email" class="form-control" id="email" name="email" required>
    </div>
    <div class="mb-3">
        <label for="password" class="form-label">Password</label>
        <input type="password" class="form-control" id="password" name="password" required>
    </div>
    <button type="submit" class="btn btn-primary">Register</button>
</form>
{% endblock %}

app/templates/items/add.html

{% extends 'base.html' %}
{% block title %}Add Item{% endblock %}
{% block content %}
<h2>Add New Item</h2>
<form method="POST">
    <div class="mb-3">
        <label for="name" class="form-label">Item Name</label>
        <input type="text" class="form-control" id="name" name="name" required>
    </div>
    <div class="mb-3">
        <label for="description" class="form-label">Description</label>
        <textarea class="form-control" id="description" name="description" required></textarea>
    </div>
    <div class="mb-3">
        <label for="starting_price" class="form-label">Starting Price</label>
        <input type="number" class="form-control" id="starting_price" name="starting_price" required>
    </div>
    <div class="mb-3">
        <label for="end_time" class="form-label">End Time</label>
        <input type="datetime-local" class="form-control" id="end_time" name="end_time" required>
    </div>
    <button type="submit" class="btn btn-primary">Add Item</button>
</form>
{% endblock %}

app/templates/items/list.html

{% extends 'base.html' %}
{% block title %}Items List{% endblock %}
{% block content %}
<h2>Items for Auction</h2>
<table class="table">
    <thead>
        <tr>
            <th scope="col">Item Name</th>
            <th scope="col">Current Price</th>
            <th scope="col">End Time</th>
            <th scope="col">Action</th>
        </tr>
    </thead>
    <tbody>
        {% for item in items %}
        <tr>
            <td>{{ item.name }}</td>
            <td>${{ item.current_price }}</td>
            <td>{{ item.end_time.strftime('%Y-%m-%d %H:%M') }}</td>
            <td><a href="{{ url_for('items.item_detail', item_id=item.id) }}" class="btn btn-info">View</a></td>
        </tr>
        {% endfor %}
    </tbody>
</table>
{% endblock %}

app/templates/items/detail.html

{% extends 'base.html' %}
{% block title %}Item Detail{% endblock %}
{% block content %}
<h2>{{ item.name }}</h2>
<p>{{ item.description }}</p>
<p>Current Price: ${{ item.current_price }}</p>
<p>Ends at: {{ item.end_time.strftime('%Y-%m-%d %H:%M') }}</p>
<form method="POST" action="{{ url_for('bids.place_bid', item_id=item.id) }}">
    <div class="mb-3">
        <label for="bid_amount" class="form-label">Your Bid</label>
        <input type="number" class="form-control" id="bid_amount" name="bid_amount" required>
    </div>
    <button type="submit" class="btn btn-primary">Place Bid</button>
</form>
{% endblock %}

app/templates/bids/place_bid.html

{% extends 'base.html' %}
{% block title %}Place Bid{% endblock %}
{% block content %}
<h2>Place Bid for {{ item.name }}</h2>
<form method="POST">
    <div class="mb-3">
        <label for="bid_amount" class="form-label">Bid Amount</label>
        <input type="number" class="form-control" id="bid_amount" name="bid_amount" required>
    </div>
    <button type="submit" class="btn btn-primary">Submit Bid</button>
</form>
{% endblock %}

app/templates/bids/bid_history.html

{% extends 'base.html' %}
{% block title %}My Bids{% endblock %}
{% block content %}
<h2>Your Bid History</h2>
<table class="table">
    <thead>
        <tr>
            <th scope="col">Item Name</th>
            <th scope="col">Bid Amount</th>
            <th scope="col">Bid Time</th>
        </tr>
    </thead>
    <tbody>
        {% for bid in current_user.bids %}
        <tr>
            <td>{{ bid.item.name }}</td>
            <td>${{ bid.amount }}</td>
            <td>{{ bid.bid_time.strftime('%Y-%m-%d %H:%M') }}</td>
        </tr>
        {% endfor %}
    </tbody>
</table>
{% endblock %}

app/templates/admin/manage_users.html

{% extends 'base.html' %}
{% block title %}Manage Users{% endblock %}
{% block content %}
<h2>Manage Users</h2>
<table class="table">
    <thead>
        <tr>
            <th scope="col">Username</th>
            <th scope="col">Email</th>
            <th scope="col">Role</th>
            <th scope="col">Action</th>
        </tr>
    </thead>
    <tbody>
        {% for user in users %}
        <tr>
            <td>{{ user.username }}</td>
            <td>{{ user.email }}</td>
            <td>{{ user.role }}</td>
            <td>
                <a href="{{ url_for('admin.edit_user', user_id=user.id) }}" class="btn btn-warning">Edit</a>
                <a href="{{ url_for('admin.delete_user', user_id=user.id) }}" class="btn btn-danger">Delete</a>
            </td>
        </tr>
        {% endfor %}
    </tbody>
</table>
{% endblock %}

app/templates/admin/add_user.html

{% extends 'base.html' %}
{% block title %}Add User{% endblock %}
{% block content %}
<h2>Add New User</h2>
<form method="POST">
    <div class="mb-3">
        <label for="username" class="form-label">Username</label>
        <input type="text" class="form-control" id="username" name="username" required>
    </div>
    <div class="mb-3">
        <label for="email" class="form-label">Email</label>
        <input type="email" class="form-control" id="email" name="email" required>
    </div>
    <div class="mb-3">
        <label for="password" class="form-label">Password</label>
        <input type="password" class="form-control" id="password" name="password" required>
    </div>
    <div class="mb-3">
        <label for="role" class="form-label">Role</label>
        <select class="form-select" id="role" name="role">
            <option value="user">User </option>
            <option value="admin">Admin</option>
        </select>
    </div>
    <button type="submit" class="btn btn-primary">Add User</button>
</form>
{% endblock %}

app/templates/dashboard.html

{% extends 'base.html' %}
{% block title %}Dashboard{% endblock %}
{% block content %}
<h2>Welcome to the Auction Dashboard</h2>
<p>Here you can manage your auctions, view bids, and more.</p>
{% endblock %}

Running the Application

To run the application, ensure you have Flask and the required packages installed. You can create a requirements.txt file with the following content:

Flask
Flask-SQLAlchemy
Flask-Login

Install the dependencies using:

pip install -r requirements.txt

Then, run the application:

python run.py

Conclusion

This structure provides a solid foundation for an advanced Online Auction System using Flask and Bootstrap 5. You can expand upon this by adding features such as email notifications, payment processing, and more sophisticated user management.