There are three great online sources for textbooks... Amazon Marketplace which you all already know about, Barnes & Noble online which has its own textbook store, and halfpricebooks.com (Abebooks) which is a repository of individual sellers and small-market bookstores selling new and used stuff. I've purchased from all three. What's nice about B&N is that they maintain all their own stocks so you don't have to rely on a third-party seller to get you the stuff. Of course, this means they have a bit less of a selection than Amazon and Half Price, but shipping and refunds are a dream through B&N.

Also, if you buy from B&N, they will make you an offer on your used book when you're done with it. Dunno what percentage they offer back as I haven't sold back to them yet. Or you can just try to sell it yourself through the other two or sell it back to your own bookstore which I do a lot to save the headaches.