Great idea. You should add a dev tag (if there is an appropriate one) to this thread.

Personally I think (if it's easier) they could make it two separate entities rather than just add a new sub ward. For example, you have the "Market Wards" and then the "Seeker's Wards", both of which have ALL subcategories and item listings. This would allow for ease of finding a buyer for an item you're trying to sell, while also cutting down on ward lag and overpopulation. By this, players are forced to choose if they want a retainer to buy (Seeker's Wards) or sell (Market Wards), and also give more purpose to having 2 retainers, one designated as your buyer and one as your seller.