Listing Fee and Price Alteration Fee!

[Jinko]

They could just put floors and ceilings on prices based on average sales price (they can get this data from the sales history), its much more user friendly than pre-taxing a player on an item which may never sell.

The more items that sell the stronger the average stability will become and the less chance of undercutting and price hiking having an effect.

Of course you would have to completely reset the markets, as much of the damage has already been done.