Online book buyers buy textbooks, and sometimes other types of books, with the aim of reselling them for a profit. Like online marketplaces, online book buyers operate year-round, giving students the opportunity to sell their books even when campus "buyback" periods are not in effect. Online book buyers, who are often online book sellers as well, will sometimes disclaim whether or not a book can be sold back prior to purchase. Students enter the ISBN numbers of the books they wish to sell and receive a price quote or offer. These online book buyers often offer "free shipping" (which in actuality is built into the offer for the book), and allow students to sell multiple books to the same source. Because online book buyers are buying books for resale, the prices they offer may be lower than students can get on online marketplaces. However, their prices are competitive, and they tend to focus on the convenience of their service. Some even claim that buying used textbooks online and selling them to online book buyers has a lower total cost than even textbook rental services.