The best time to sell your textbooks is from December 1-13. This is the time of year to get the most money for your books. You can sell them back from any semester.
If you have a Yellow, Introduction to Hinduism and Buddhism, by Professor Chakrabarti, stop by the campus shop before December 5th to sell your book to get 50% back. The sooner you stop in, the better, while this offer lasts.