I cannot say it enough - I love Amazon. This is one company that has not managed to tick me off or disappointment me. I can't say that for my current cable company but we won't get in to that right now.
I recently shipped my daughter's calculus book to Amazon Textbook Buyback and they gave me a credit of $136 - we paid $200 at the school book store. I did not even have to pay for shipping. I printed the shipping label Amazon provided and all I had to do was pack the book up in a small box, tape the shipping label on it and drop off at the post office. It was that easy. My Amazon credit never expires.
There were other sites willing to pay me cash but not as much as the credit at Amazon. Every book is different price-wise and you just need to look for the ISBN # on the back of the book and enter it for the buyback amount.
I just wanted to pass this on to all my friends and family that have kids going off to college. Well, I have a couple more books to look up, so I better get started.