It is a good idea, I agree. I want to know if the person I'm dealing with is reliable and fair.
The problem is - and this is probably a conversation that needs to happen in the admin forum before we do something like this - there is a danger of it becoming a forum for grudges and character assassination. Apparently eBay found the same, hence their changes to their own feedback system.
Of course eBay is much larger than us and so harder to police, so we should be able to manage. It's more a question of what liabilities we would have if we allowed grievances to be aired, and whether "User X sold me Y and it was rubbish" should be allowed to stand without verifiable proof.
Maybe as a trial we could have a subforum for buyers and sellers to comment on each other, but only in a positive way?