Honestly, I'm surprised authorities haven't come down hard on these 'marketplaces' for neglience given how often they seem to completely ignore both safety and IP laws.
I guarantee a smaller company would probably be sued into oblivion if they were as relaxed about what they stocked as Amazon is. Same with an app store that was as willing to stock knockoffs and fakes as the iOS App Store and Google Play.
The fact these companies seem to be able to just stock anything and everything without any sort of oversight or quality control, and can just basically say "buyer beware" boggles the mind, especially compared to traditional retail and offline equivalents.