I worked in a college bookstore for several years and there are many factors that go into what the store is paying the students for a sell back book. Factors include how long the edition has been in print, condition of the book, was it a consumable book (workbook, lab manual, etc).
The best time for selling is during finals because that is when the bookstore has a need for books. It is very important that professors notifiy a bookstore for the next sememster - if a professor orders the book for the next semester the bookstore will pay 50% back (i.e. $100 book has a resale of $50).
If you sell off of the finals schedule, the price is based on demand for the book nationwide and is usually 15-33% of the original cost. If the publisher has a new edition in the wings the old edition has 0 value. These books don't stay in the store and they are shipped out to a warehouse for distribution to a school that has orders for them.