add an option that for every transaction you make at the market you get a transaction paper with the names of buyer seller, what has been sold and bought and the date. when you do a foul-deal or miss-deal you can go to a moderator in a sub-board at the market, where you can make a topic about it and show you transaction papers. this way the moderator can diside if it is true and really a foul-deal. and than the moderator can un-do the transaction where both players get their stuff/money back.
though a few things have to be made:
-the site update
-a sub-board in the market
feel free to post and discuss
Poll