Yes. Trust rating is broken. It should have only trade deals! But everyone leaves only their comments, although they did not make real trades.
The problem with not allowing people to post feedback unless they have traded, is that in order to give someone negative feedback, you would have to be scammed first. The goal is to prevent the scams in the first place not to just stop it from happening again. If I see someone trying to pull off a scam, should I have to let myself get scammed first so I can warn everyone else? No. I should responsibly disclose why I think they are scamming, and allow people to judge for themselves the validity of my claim based on the evidence I provide.
If you did something that makes you look untrustworthy, such as selling your trust to another, anyone is more than entitled to give you negative trust and to point that out.
Every scammer has positive trust before he deceives people. You can not prevent it!