3) Nobody should be able to "rate" another user without having done business with them directly, and it would not be too difficult to create a system where two or more users agree to a transaction and allow themselves to rate each other. This record could then be publicly displayed.
So we wait for an obvious scammer to scam someone before the person who gets scammed can leave trust which will then be pointless as the scammer will have just ditched the account and moved on? This method would only really benefit scammers who would be quick to take advantage of it.