There are many websites where you can buy used textbooks, such as Chegg.com, Amazon.com, Half.com and CollegeBooksDirect.com. Some of these websites allow you to rent new or used textbooks, instead of purchasing them. Renting textbooks is cheaper than buying them.
Chegg.com is popular for allowing customers to rent textbooks, but you may buy used textbooks there as well. You may buy or rent used textbooks on Amazon.com. Alternatively, you can purchase an e-book, which is cheaper than purchasing a new physical textbook.
Half.com offers purchase and rental of used textbooks as well. Users can also list their own used textbooks to sell them on Half.com. CollegeBooksDirect.com is similar to Half.com in this way, but is geared specifically toward college textbooks.