If your school has a bookstore, you can usually sell your books back there. You may not get the best price for your books, but this is a quick and easy method to trade your textbooks for cash.
Go to Textbooksnow.com. On this site, you can search for your textbook, and the site will tell you how much you'll get for your book. If you like the price, go through the steps and Textbooksnow will provide you with a shipping label so you can ship them your book for free. To be guaranteed the price you were offered, you'll need to send your book within ten days. Soon after, you'll receive a check for your books.
Know that your books will have to be in good condition to be accepted by Textbooksnow.com, and you won't get your money or books back if they don't accept them.
Sell your books at Half.com. Here, you can choose your price, but you are responsible for shipping your book when someone buys it. You'll most likely send it Media Mail at the buyer's expense.
You can choose the price to sell your book for, but be aware that there will be competition, and unless you choose a really low price, others will likely sell their textbooks for less money.
You can also sell your textbooks on Amazon.com. Amazon.com is good because a lot of people view the website. Know that like Half.com, lots of books are listed at varying prices. Also like Half.com, your book can be in any condition - but you'll have to admit that condition on the website.