Half.com

Perhaps the best idea I've ever seen on the web. It offers a marketlace to buy and sell used books, CD's and DVD's, and as such borrows from places like Ebay.com, but rather than take bids online, there are set prices for purchase.

What really makes this smooth is that Half.com handles the financial transaction and mails the proceeds to the seller.

I recently needed to purchase 'Impro' by Keith Johnstone for an improvisational comedy class I am taking at Unexpected Productions, so made use of their service. Locally, it would have cost me about $20, but I got it from an private seller for about $9.

They also did a publicity stunt that resulted in Half.com, Oregon.

...and while you're in Oregon, if you'd rather visit a bricks and mortar shop, try Powell's City of Books

This is a fantastic site, but you have to know how to use it. What I do is, go to Amazon.com and use all their neat recommendation engines, user book lists, reviews, etc and find books I'm interested in. Then I go to half.com and look for the books there. It is almost always cheaper, even including shipping, which usually accounts for something like half your bill. You can cut down on shipping if you buy more than one book from the same person, but this is somewhat difficult to do.
The only real problem is that it can take 2-3 weeks to get your books. But if you're in no rush, then Half.com is a great way to buy books and more.
Amazon now has a feature like this on their site, but I don't think they have quite the user base Half.com does.

You can also buy books (computer books work well) from a discount catalog like Edward R. Hamilton and sell them on half.com at a profit. BTW Half.com is owned by ebay.