I've already thought of a scoring based system, but I haven't had time to implement it yet. My idea is to include a scoring system where you would rate each person/identity on a percent trust/distrust scale. Everyone would start out at zero. As you complete trades with someone, you can increase your trust in them by raising the scale to a positive percentage. If the trade goes bad, you can decrease the scale into the negative territory.

Then, all of the trust ratings from the people you already trust would be combined with how much they trust someone else to determine your expected trust in an unknown party. I have an idea how to do that, but I don't want to go into detail here.