A middleman service with a standard API plus the push from consumers / front-ends might encourage them to build that out. In any case, I wasn't thinking individual meatspace retail stores; I was definitely thinking more "whatever is sold on the web site now" (e.g. Macy's, Asos', Urban Outfitter's online stores - which I'm making the hopefully-not-wild assumption have their own separate inventory.)

Tackling per-physical-store retail inventory with the challenges you mention is definitely a whole other thing.