There is no universal solution. Every jurisdiction will have different requirements that must be followed to do business above board. There can of course be in person cash transactions, but anything digital will likely have to follow laws, and the laws will vary widely.
We're talking about decentralized exchanges, so the law will have to sit aside.
The issue is more having trusted escrow of the BTC using 2 of 3.
I think the solution is a web of trust, when someone has a large amount of trust they can be an escrow agent. But only for the BTC part of it, which uses 2 of 3 keys. This way he can't steal.
The only issue is what if I get a trusted account, and keep posting trades until I get myself as the agent.