This is a fantastic site, but you have to know how to use it. What I do is, go to and use all their neat recommendation engines, user book lists, reviews, etc and find books I'm interested in. Then I go to 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 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 does.

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