A common thing amongst most MMORPG\'s is what you said, and is a good idea but it shouldn\'t replace player shops in my opinion.
Sometimes when you leave a msg to buy something or sell something, you then have to check back, see if you won it, whether or not you need to bid more, etc etc. Sometimes you won\'t even have the time to pick up the item and it is often a long process.
Say if players could rent or buy their own shops and put any items they had in it, and put a price for them they liked. That way if anyone was interested in buying an item another player had, they could just buy it from the shop and the player who owns the shop would automatically get the money credited onto their account so that next time they log in they will have the money.
Perhaps shops could be like this -
Small Shop - Rent: 2000GP/week Buy: 80 000GP
Large Shop - Rent: 5000GP/week Buy: 200 000GP
Warehouse - Rent: 10 000GP/week Buy: 400 000GP