Sell the books back to the original bookstore you bought them from, if possible. This usually works when you've purchased them from the school's store or a local bookstore. Find out how much they are paying for them.
Check out a few online bookstore websites. These include half.com, chegg.com and even ebay.com. To do this, you may have to set up an account. Then look on the sites for a "Sell" button. Chegg does not always buy books from students, and eBay may not have a good market for books until the beginning of the term (fall or spring).
Sell directly to another student. Post fliers around your school with a picture and the year your book was printed. You may even want to add the ISBN number. Give the price and a way for the student to get in contact with you.