One problem with the trust system that would be great to try and solve it retaliation feedback. Often times people are afraid to leave someone who they think is untrustworthy negative feedback because that person has DefaultTrust and a high trust score and they fear retaliation negative feedback. I'm not sure of a good way of mitigating this, but it would be very useful.
IMO, removing the default trust list, and creating some sort of system to have BOTH users agree to a transaction before any trust ratings that can effect your trust score can be left would go a long way in doing this. People would still be able to comment on your trust page with neutrals, but it would not impact your trust rating. This 100% ensures only directly involved individuals in a transaction have the power to damage another persons trust score. Furthermore it may even be good policy to require users post the agreement first and reference during this mutual agreement function it so any disputes can be arbitrated based only upon the agreement the two entered into.
Then what happens when I agree to a trade with a newb and he doesn't pay or tries to scam? I leave negative then he returns the favour? This retaliatory feedback is what used to happen on ebay but now only buyers can leave feedback but then they often just leave negative for the slightest of things and it'll happen here. You can agree to sell something to a newbie and then what's to stop him from leaving feedback for no valid reason? This system also won't stop scammers and warn others before they scam and it's too late after they do.