> the big five publishers theorized that third-party sellers might be selling some of the free promotional copies… which have no such marking on their covers and often end up sold to bookstores like the Strand.

If these promotional copies already being sold by other bookstores then what's the problem? If they shouldn't be sold then the publishers should clearly mark them as not for sale.

> Others have suggested that they might be buying books with minor cosmetic damage from warehouses…

Presumably these slightly damaged books were originally printed and sold by the publishers too, so what's the problem?