Something like that safe trade thing they have in lelong.com i think (can't remember how it goes exactly)

Been suggested before, but yeah in order to implement this you'll need the 3rd party guy to charge a fee (who's gonna do it for free?)
Then there's the problem of who can actually be trusted to be the 3rd party, nice opportunity to scam someone without them even seeing it coming

If you feel the need to and have the determination to set something up like this we could allow and try to help out, but i personally am too lazy to be 1 of the 3rd party guys