Perhaps this information is coming a little late for all you fellow textbook-owners out there, but I have to share my love of Amazon textbook trade-in with you (and no, Amazon is not paying me to talk them up).
How it works is that you start with a textbook that you never want to see again. You go to the textbook trade-in portion of Amazon (books--textbooks--sell your books). Type in the ISBN of your book. Select the right one from any options that come up (usually, there won't be options since ISBNs are unique--it'll just take you straight to the book). Then you can select to trade-in your book for whatever price they list (I've found it's usually a pretty decent deal). Then it takes you through a "check out" type experience, where you select the address you're shipping from and what carrier you want to ship with, etc. Then ultimately, it gives you the shipping label (at no cost to you). You print it out, slap it on the box with your books and drop it in the mail. After Amazon receives your books they will credit your account with however much money in Amazon gift cards.
So, to sum up. Amazon will take your unwanted textbooks off your hands, at no cost to you, with comparatively little inconvenience, and they will give you Amazon money in exchange.
Oh wait, it gets better. Since Amazon sells books to people everywhere if you can't sell your book back to your local bookstore because a new edition came out (or some other equally stupid reason), Amazon may still take it.
What spurred all this was that last night I finally got around to taking care of my textbooks from last semester. I managed to sell both of them back, one for a pretty fair price (I bought it for $60 and sold it back for $40) and the other one I made a profit on--I bought it for $20, and sold it for $22. Ok, so maybe $2 profit isn't much, but in the world of textbooks that's huge. When was the last time you sold a textbook at profit?