Perhaps the easiest way to sell used text books is to sell them back to the school bookstore. Almost every school bookstore will allow you to sell used books, as they can then sell those used books back to other students (for a higher rate than you sold them for). Most school bookstores have textbook buyback at the end of each semester or quarter. So in order to sell used text books, just take your books to the school bookstore during text book buyback and sell them for cash.
There may be other bookstores at your school, besides the school bookstore, that will buy back books. These bookstores may offer high buyback rates, so you might be able to get more money when you sell used text books. Try to find private bookstores around your school that buy used text books.
Another option is to sell used text books online. When you sell books online, it will take longer for you to get your hands on the actual money. However, the amount of money you can sell used text books for is generally much higher than you can sell them for at an actual bookstore. This is because there is no middleman when you sell books online.
You can sell used text books online at websites such as Half, eCampus, Amazon, Big Words, and many other websites (linked in the "Resources" section below). Some of these websites will even pay for shipping! Whatever you choose, whether it is to take the quick cash by selling used text books to a bookstore, or wait and make more money by selling online, now you know how to sell used text books!