I second both campusi.com for buying and half.com for selling. Campusi.com saved me a ton of money since I was led to it, although my school's bookstore is sometimes competitive with the prices on its used books. Half.com is easy to sell too, I sold my Botany textbook for $65 in 2 days, compared to the school offering to buy it for $19. It only cost me about $70 to buy it new thanks to campusi. I've already sold most of my books, but I have a couple used anthropology books, religion, and a biology book, all in new shape. If someone lists one of them as something they need, I can make a deal. These are all a little more obscure and I doubt many classes will require them, but I just want to sell them cheap as long as its more than the $1-2 dollars offered by the school.