You can get some really good deals on Half.com too... but find out what books you're going to need way in advance if yoiu're going to buy from any online service.
Barnes and Noble's website (
www.bn.com) and
www.ecampus.com are great places to sell your books back to... they give you a decent amount for them and they pay for shipping. I know you get more from them than you do selling them back to your bookstore, because I tested it once... I checked out how much I'd get selling them at those websites, and then I took the books to the campus bookstore and found out how much they would give me. I got more from the websites
The only disadvantage is that you don't get instant cash, you have to wait for them to mail you a check.