I think it would be very interesting to write down a standardized guide, governance and techniques to enable this. @nout's basic rules are a fantastic start. But, how does canceling an order work? What happens if a fill isn't marked filled soon enough? What happens if somebody robs somebody else? How does adding some body else work? Do you have to level up, do a few trades before you are allowed to trade more? Is there a public history of who trusts who? What happens if the bank screws the fiat side of the trade? So many more questions...

If it was codified, written down, into a potentially versioned standard then protocols, scripts, and tools could form around it. And if you joined one p2p exchange, then you could join a second, third or fourth one without learning how it worked.