To be completely honest, trades are always a huge shitshow (with or without the assistance of an OMM). I'm sure it's less of a headache not involving someone directly with the trade. MPGH isn't liable for anything lost during a trade anyways. The only think keeping the OMMs from stealing from the people participating in the trade is their MPGH accounts / reputation.
My opinion on the matter is simple, if you want to risk trading your accounts on a forum advertised for hacking, then you should at least know how to avoid being scammed. It's really not that complicated. The MM usergroup was a fine idea, but there isn't a perfect way to implement an OMM into every sort of trade, especially when you consider that each MM doesn't have experience on every game currently on the market.