Everyone is talking about price limits when you should be talking about time limits.
Limit the amount of time someone can sell an item. Think of how Ebay works.
We do not know the asking price, but the amount of time an item can be on the market for sale can be controlled. If the item is on the market goes over the limit of time, then the item is pulled off the market and the person will have to pay a fee again to post it for sale.
Limit the time an item can be on sale for 72 hrs. and everyone will have to try harder to sell their items.
