Your plan limits people to selling things on retainers still, basically forcing us to use the limited capacity wards, and it relies on items being sent to mailboxes, a system that is not simple, or time efficient. This hurts the AH side.
It also takes out pretty much any reason to go window shopping, because anybody with a brain (or without 30 minutes plus to try to find a few items) would use the buy it now function, thus negating the need for retainers in this system at all. They would only serve to limit the sellers, and waste people's time by forcing us to set up the retainer in a ward, and go back and check it whenever we want money, or to exchange items with it.
This could all be handled by an auction house NPC instead, so the process of buying and selling would be streamlined. The Wards would just be there for anyone else who really wants to use them (or if there were items that could only be sold on characters/retainers, they'd be sold here, like XI's RBF).