Here's an off-the-cuff idea. Before a peer-to-peer trade both Alice and Bob send a signed message to each other. Alice signing "I have initiated a trade X for Y with Bob". Then after the trade, Bob countersigns the message with "I successfully completed this trade with Alice" or "Alice didn't live up to her end of the bargain." These can be tallied up based on how close they are to you on the network for a score.