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If Amazon stocked every book it sells, it would need a warehouse the size of Nevada. It would also have gigantic inventory costs. Amazon uses three basic methods for selling books (1) standard inventory; (2) "just in time" inventory; and (3) third party sellers. Amazon has a physical inventory of the most popular books it sells. They are in a warehouse and shipped promptly when somebody orders them. Less popular books are subject to "just in time" inventory management. Amazon doesn't have the books, but has arrangements with the publishers to ship the books as soon as it receives orders. This results in a slight delay in shipping to the customer, but Amazon saves a lot of money in inventory costs; it doesn't have to store the books or pay for them until it sells them. Finally, Amazon has a network of third party merchants who sell through Amazon. These third party merchants generally specialize in out-of-print books or books that deal with relatively arcane subjects. Those books are shipped to the customer directly from the third party and are billed by the third party to the customer's credit card or paypal account. Amazon takes a small commission for acting as a middleman.