Sell old edition textbooks. Even if college bookstores won’t buy back certain books after the semester, check with other bookstores to see if they will purchase them. Also, there are several different online markets in which to sell books, such as Amazon.
Donate the books to charity. Give your old edition textbooks to Goodwill or a service organization; they will distribute books to prisons, alternative schools and libraries. While you may not receive direct money for this action, you can write it off on your taxes, but only for the actual value of the book at the time.
Use the book. For some classes the information does not change; just the pictures or illustrations are updated. Chapters may be reordered. However, for highly technical classes, there is a chance the information could be substantially different. Never try to use more than an edition lower of what is assigned, and double-check the new edition versus the old for accuracy.
Keep as a reference. Some college students want to keep certain books for future reference in their careers. With older editions, the information won’t change dramatically and can still be helpful several years later.
Pass along your older editions to friends. Oftentimes, students are stuck with a textbook when a university decides to use a new edition the next year. Find a friend who needs to take the class, and sell it to the friend for a reasonable rate. You make a few bucks and the friend saves a few dollars buying an older edition textbook.