Most campus bookstores buy back your books at the end of the semester. Timing is everything. Get there before other students do, and you will most likely get better prices. Of course, they won't buy back extremely damaged books, but even if your book is in perfect to near mint condition, you will not get what you paid for it. The store may not accept all of your books since new editions come in or they already have too many, which will require you to turn to the other two methods.
You can sell your books on websites like Craigslist.com, Amazon.com, eBay.com and Textbooks.com. Some online college bookstores will even pay for the shipping of your books, and will give you a flat rate before you even send your books to the buyer.
If you walk around campus and notice the bulletin boards, you'll spot some ads put up by students looking to sell their books. This is a great way to sell your old books, because you don't have to hassle with trying to get good prices from the bookstore or shipping them. All you have to do is take a picture of the book, describe its condition and set your price.