That actually doesn't sound like a bad idea on paper. Combining this system with a more traditional auction house search function would make utilizing the current retainer system a much more strait forward affair. The only thing that sounds like a problem is the summoning portion. Summoning and dismissing retainers could clutter up the hub more than it needs to be and make exchanges that much more difficult to complete.

A good twist on this would be to have a UI pop up and allow the player to interact with some kind of NPC organizer. The player finds what he is looking for in the search option, the NPC organizer goes off to call up the seller, and then brings the seller to the selling window where the player would interact with him. This will help keep the exchange area a bit cleaner and still provide the same service, since the NPC isn't really being summoned to the zone, but rather to the individual players UI window. It also helps prevent having several duplicate retainers out at once, unless we all want to believe the retainers have learned how to clone themselves spontaneously...

Actually, self cloning retainers sounds awsome. We need more self cloning helpers.