I don't know how each college/university does it, but I find the bookstores NEVER give fair value for the books. I put postings in the areas of the school to sell the books. I am also involved in a book exchange where you can exchange books for classes instead of having to buy new again. It has worked very well. Usually the classes involved are elective classes, but it still has saved me money otherwise.
Otherwise, I keep all my books for my nursing major. They are SOOO expensive, and I am able to use them as reference for all my future classes.
Jo