I've had good results selling books used on Amazon.com, although it does take time for someone to want to buy them.

Alternatively, if you're willing to just trade them, check out www.bookmooch.com . It's a site that lets you list books you don't want anymore, then other people can "mooch" them. When someone mooches a book from you, you get a point, and you can use that point to mooch a book from someone else. Also, you get 0.1 points just for *listing* a book, so if you list 10 books, you get a point right away, so you can mooch a book without waiting for someone to mooch one from you. Plus you can import a wish list from any text file or html file or URL that contains ISBN numbers, and if a book on your wishlist becomes available on bookmooch, the system will send you an email.

It's a super cool system and I have been having a lot of fun with it. (But if what you really need is cash, it won't help I'm afraid.)