Why not have an auction house in each city?
I would see it working the following way:
I go to the auction house, and when I enter, a screen pops up. I can then choose one of my items, set a starting price for it, and set an amount of time in hours for the auction to last. The item then leaves my inventory and is held at the auction house.
People visiting the auction house could submit a bid on my item. When they submit a bid, the money is removed from their inventory, but as soon as someone out bids them they get their money back. The first bid has to be at least the starting price I set. After the preset number of hours has elapsed, the winning bidder has to visit the auction house and when they do so, they recieve their item. If the winning bidder doesn\'t collect the item within thirty days, it is lost forever, but the seller still keeps the money. The auction house would keep 5% of the winning bid, the rest of the bid would be given to me next time I visited the auction house.
If no bids are made on the item after the set number of hours, then the seller can return to the auction house to pick up their item.
The ideal user interface for it would be that when you enter the auction house a window pops up with two buttons, buy and sell. Clicking the buy button brings up a list of all the items currently up for auction. Idealy you would be able to search for a particular item, or items put up for sale by a particular person. It would also be nice to be able to sort the list of items by price, category, and amount of time left on auction. The row in the list for each item would show item name, category (weapon, armour etc), person who put it up for auction, current winning bid, amount of time left for auction and a comments field. If I selected an item, a box pops up asking me how much I want to bid. My bid must be greater than the current winning bid, but not more than the amount of money I am carrying.
If I click the sell button, then all the items in my inventory appear on screen. I can select one of them, which causes a screen to pop up where I can set the starting price, the auction length and some comments to appear next to the item.
Some more advanced features could possibly be auction multiple items in one lot, recieving a message when you are outbid, and automatic bidding, where you tell the auction house to bid for you up to a certain amount.
As you can see I let the auction house take a cut of anything you sell, so it will be more profitable to sell by a normal trade, but the auction house lets you sell things even when you are not online.
EDIT: Fixed a loophole
