This website http://thriftbooks.com/ has used books very cheap -- some as cheap as 1 cent.
The catch is that you need to pay shipping for each individual book. First book is $3.95 and each additional (of the same order) is $3.49. But that's still a deal if you are looking for a certain book and can't find it anywhere else for a good price. Used is as good as new when it comes to books, unless they are written on or stained.