Lol about the black market.
My issue with your proposal is that it takes away the possibilty of people having their own stores, or any possible game ideas that might be associated with that. Plus I really love my retainers.
Another thing that will happen (and one reason I don't care for AHs), is people all crowding around one particuliar NPC, which isn't very market-like to me. It also takes away the shop experience that some of us like.
I think your solution is good, but it won't address certain things.
How to let people have their stores and keep it organized, I admit that un's a biggie.