I think that mandatory middleman service will cause LOTS of hassle, and eventually slower the fuck of the marketplace.
I agree with the opinion to temporary close threads until receiving vouches.
However, there is one thing to all that.
In my view, most of the thinkable measures can only be aimed at the low-reputation people and so on.
Just take the example of Vaypah. Even if MPGH limited the amount of orders, nothing would preclude him from completing the first ones, getting increased limits and only then scam.
Honestly, I can't come up with a solution to protect users from reputable scammers.