If there are 100 copies of a book for a class, half new and half used, then the bookstores will need to have 50 used copies at the end of the semester (so they can offer them next time. There are 100 copies in students hands, but the stores only needs to buy back 50, so only half the students can sell their books back. If you don't hurry, you lose your chance. (Unless you sell back online).