Most university bookstores offer buy-backs. While you probably won't get a full refund, you make some quick cash.
If you have the option, you can also rent your books online. Be careful not to pay for something that is only a study guide or an online version, but websites like Chegg.com or Amazon.com can save a lot of money even if you do have to pay for shipping.
Have an e-reader? Use it for class. Textbooks are sold for discounted rates and you don't have to worry about what to do with them once the semester ends.