Another forum I am a member on has one section where each thread is a member's name, and in that thread you are supposed to post feedback about sales and trades. No other posts are permitted. Then if you want to trade with someone, just read their feedback thread.
There is a possibility that a system like that would be open to abuse e.g. people claiming something nasty against someone they don't like, and disputes might occur if one feels cheated and the other doesn't see it the same way.
But that other forum is much smaller than GH and most members are very well behaved. (Not saying GHers are not well-behaved, but there have been a few who sow dissent amongst the ranks).