Most of the books in a major bookstore like B&N are returnable. They're not actually owned by the bookstore (at least not permanently, and not outright). If the book doesn't sell within a specified period of time, the publisher will take it back.

Hardcovers are generally returned intact. Paperbacks and magazines often just require that the retailer tear off the cover and return that, while destroying (at least in theory) the rest of the book. That's why you sometimes see that warning in the front of paperback books and magazines about not buying it if its cover has been removed... unscrupulous retailers will sometimes strip the cover, return it for credit, then sell the stripped book on the black market.