My idea stems around the similar function of the Second Life developer shop. Each player can open their own shop and upload their own items for people to buy. Only difference is that the players would be responsible for keeping said item in stock or removing items from their storefront as necessary.
Additionally it would allow for more transparent pricing and viewing to see who has the best deals on items. I also think it would be a cool Role-Play aspect to the game to let people really dive in on the merchant path.
I'd be willing to expand onto this concept.