The scam itself may be clever but it still relies on 2 people doing trades with untrusted members or failing to use ;;ident properly, so it's easily avoided.
If people fall for this the scammer could have achieved the same result by just buying bitcoins with paypal and doing the chargeback himself.
but in this case the scammer would have to handle the money and is traceable to his paypal. I'd assume the reason MITM is done is to be practically untraceable, especially as BTC is anonymous