Cool idea, but this would never happen because sellers would hate it. One of the main reasons eBay sellers brand their images with obnoxious image graffiti (overlayed text and/or styling) is that competitors have no qualms about stealing images. eBay plans to formally restricts the use of image graffiti later this fall (http://pages.ebay.com/sellerinformation/news/newlistingrequi...). I doubt that a seller who takes product photography seriously would like to hear that their competitors are legally entitled to use their photos, regardless of whether or not the practice is opt-in.