@bbsbazaar
Every week, the major retail booksellers distribute "pull-sheets" to their lead employees or department managers, who then remove all of the titles on those sheets (by ISBN), to be returned to the distributor, who then returns them to the publisher. The retailer receives a percentage of the original cost which was paid to the distributor or the publisher.
The publisher re-sells these book returns to a book warehouser, who then slaps a new ISBN number on either the front or rear of the book; and many of these "new" books are then re-shipped back to the original retailer, who then displays them on their "bargain" shelves.
After a few months, new "pull-sheets" are published, and those ISBN numbers are returned to the book warehouser, who then fills multiple boxes with miscellaneous returned titles, and builds individual skids filled with boxes of books to a certain height, and then wraps sheets of plastic around the individual skids.
These skids are auctioned off as "blinds" to "thrift" type booksellers, who may have spent only pennies per individual book -- thus permitting the "thrift" type bookseller to earn a small profit on each book, even with "free" shipping.
It's nearly impossible to compete with the "thrift" type bookseller insofar as contemporary book titles are concerned -- so it's far easier to attempt to sell book editions which are long out of print (meaning pre-1985).
