if a textbook is used in subsequent semesters and is not part of a package (and sometimes if it is), bookstores will pay decent money for them. If not, they go to "wholesale" or "speculative" prices, which is what everyone complains about. Unfortunately the life of a textbook is maybe two years now, so my advice is to sell it while you can. If an instructor is slow to readopt a book, then a college bookstore won't know for certain if it's used again and it won't be on their buyback lists. There are lots of factors that affect when to sell a book, but if it's used again, the price will be as good as it will ever be.