@ The problem with only holding one part of the trade as an middle man, is that you can't be sure what has happened to the other part of the trade.
Party A says he has gifted the ship, Party B says he didn't receive it. Now who do you send the cash to?
Only way I would consider being an middleman is if i can guarantee both party will comply.
There is an workaround that will require even greater trust in the middleman. Party A sends the cash to the middleman, Party B sends his login on RSI to the middleman, Middleman gifts the ship in question himself from Party B's account, Party B changes his password, Middleman sends the cash.
I don't think many would favor this kind of trust in a middleman, but the way I see it that's the only way the middleman can guarantee fair play from all parties.