Do I really need to logically disassemble your statement, even though I never said anything provoking your response? Ok, I'll bite.
'A trusted middleman'
1) Whose middleman? Yours, his?
2) If his, why would you trust his? If yours, why would he trust yours? If a stranger, why would either of you trust him?
3) 'How is he gonna scam you if...': Insert any situation for the ellipses and I guarantee it has been done or will be done in our human history.
4) The only way your suggestion is even applicable is when all parties already know eachother.
Steam trades work as you can see what you are getting in real time, much like how trade windows work in MMOs. But by all means, please provide some more carefree and poor suggestions for people. Need to refill the pool of Nigerian Prince subjects.