Here is the problem with the whole idea. What happens you you call your retainer? That is right, you no longer have anything listed for sale from that retainer. When you call it, you get a message saying "You are no longer selling items on the market". What you propose would only work in games like GW2, FFXI, etc with real auction houses. Until they rework the whole retainer market board system, you won't see this implemented. The sad part, they are not even listening to us, at all, not one, damn, bit. I have seen many of these, and no one from SE has anything to say on this matter. Not that I have seen anyways, and I would love if someone could prove me wrong.