I used to sell used college text books to the campus bookstore but was never satisfied with the amount I was paid for them, sometimes it was less than 10% of the list price and if a new edition was going to be used they wouldn't buy it at all.
Now I sell all of my text books on Amazon.com and have had far more success. On Amazon I typically get 70-80% back from what I originally paid. If priced correctly the book will sell on Amazon within a day or two.