So basically you want to implement an elaborate system so someone can't undersell you? As mentioned people WILL find workarounds. What you have here is telling people they can't eat steak because a baby can't chew it.

The deal breaker here is the game/NPC's enforcing a price on items, that will never fly. I would like to see retainers replaced by item specific NPC's though. Kind of like taking the FF11 AH and splitting the NPC's up into weapons, light armor, heavy armor, crafting material, etc. you keep the ward feeling and the efficiency of a AH without just copy/pasting FF11's system. Saieno, Errn and I (Neiloch) actually talked about this a bit in our podcast on lodestoneradio.com