Sell your books back to the college bookstore at the end of the semester--but only if your sole priority is unloading the books. Campus book buy-back programs usually pay the lowest of all possible options for selling your books. If you need the money immediately, this is your best option.
Sell directly to other students at the beginning of the semester. You can post notices on school bulletin boards or on the door of your dorm room. Most colleges will have a specific policy on how to go about selling text books from student to student. If you aren't sure of the rules at your school, ask other students or ask someone on staff.
Many larger colleges have off-campus bookstores very close to the school. These stores offer used text books at lower prices than the college charges. You can sell your books to these bookstores for slightly higher than what you would get on campus.
Donate your books to a library, second-hand store or charity. While you will not get any cash for the books by doing this, you will be able to take a charitable donation deduction on your income taxes. If your parents actually paid for your books, save the donation receipt for them. The deduction will probably be more useful for them.
Ebay is a common place to sell books online. Create an account and start listing items. Remember not to start the bidding price too low on your books because if only one person bids, you may end up giving away a $50 text book for $1.99. Be reasonable when setting a starting price. If your sale prices are set in stone, use the Buy It Now feature instead. If someone likes your price, she can buy without bidding. Money made on eBay must be deposited into a PayPal account.
Another useful site for selling text books is called Half.com. It is a sister site to eBay but it works a little differently. You list the books you have for sale and give them a set price. The shipping price is set by Half.com. At the end of the month, you will receive a payment by direct deposit into your checking account for all books sold and as reimbursement for the shipping costs for books that you have mailed to buyers.
A new start-up company called Chegg.com also offers you the opportunity to sell your text books. Unlike eBay and Half.com, Chegg purchases all of your textbooks upfront, then lists the books on its website. Students use Chegg.com to rent textbooks for the semester and send them back when they are done. Chegg even pays for your shipping to send your books to them.