I like the idea of mixing the Retainers and Menus together.
Player has a Retainer. They give their items to the retainer to sell.
Player goes to Market Window. Searches for item.
Finds item in item list, shows retainers selling the item and for what price.
Open the retainer, the item is marked, buy the item, window shop the rest of the retainers good.
Log out of the window.