Bear in mind the shopping habits of the average used book buyer. First, he's a browser. He notices your shop, drops in and begins looking around to see what kind of books you have available. If he spots something that really interests him, he'll probably buy right then and there. Ideally, a used book store will need a market population of at least 50,000 to support it.
Shopping malls are excellent locations for book stores.
If the buyer doesn't find what he wants in the other book stores he'll check your store. Grocery store shopping centers are generally poor locations for book stores of any kind. It's important that there be a lot of casual strollers in your location area and that you encourage these people to drop in and browse around. If you want the entire front of your store to be a show window, take pains to arrange your display in an uncluttered manner. However, a window display is not a necessity.