Selling back your textbooks  

Everytime I try to sell back by textbooks at the end of the semester, I usually take them to the campus bookstore and recieve not even half of the money I paid and as college student, I need all the money I can get. Just yesterday I decided to try something different and put a book up on Amazon.com, it sold within a half an hour for a substantial amount of money (more than it would have sold for at the bookstore definitely). I also saw that half.com or ebay were recommended places to sell textbooks. Where do you sell back your textbooks to get the most money for them?