It is illegal to sell a "stripped" book because it is common
practice for booksellers to return the covers of unsold books to
the distributor to claim a credit for the book. Just the covers are
returned to save on shipping costs, with the assumption that the
now-unsalable remainder of the stipped book will be recycled or
otherwise disposed of. If an unscrupulous bookseller then tries to
sell the coverless books, they are essentially stealing from the
distributor.