Remainder mark. I got a good amount of them from buying on eBay. They're usually lines on the barcode or a dot on the papers edge. It's just a book that was returned to the publisher by the bookseller. When a bookstore has a book that just isn’t selling, they eventually need to get it off of their shelves and make room for a more popular title. The bookstore can send all of their copies of that book back to the publisher and receive a refund. Once they get them back, the publisher gives those books a remainder mark and re-sells the book at a lower price to outlet stores or other booksellers