Flippa is the world's best known market place for buying and selling websites. It has a worldwide reputation and if you know what you're doing can be very profitable.
They are currently offering a free e-book when you sign up, which will show you how to:
* Find Websites to Buy
* Assess Sites for Investment
* Negotiate the Sale
* Adding Value to Websites
* Preparing Sites for Sale
* Getting the Best Price
Be careful when purchasing anything online and make sure you know what you're buying!