I also like the idea of neutral feedback, or no feedback being considered neutral.
Ebay uses a similar system. It lets users see what percentage of feedback was positive or negative.
To me, the "+10, -1" example presented by Sebi is a good start, but if we are going to have neutral feedback, I'd also want to know that number too. Not necessarily as another number showing feedback ("+10, 3, -1"), but simply as the total number of completed trades (e.g., "14 trades, +10, -1) or as the percent of trades with positive feedback. Given two people with a feedback score of "+10, -1", trading with someone with that score out of 15 trades is much less a risk than someone with the same feedback score and 30 completed trades (showing that half of the transactions were neutral).
Then, of course, there's a big difference between someone who has one negative feedback out of five trades than someone who has one negative feedback out of 500 trades. I might avoid trading in the first example, but would have no qualms about trading in the second.
I believe most users would like to be able to review the reasons listed for feedback (negative or positive). Looking at lessons learned by Ebay, however, we might find that traders may eventually want another field that will let them respond to any negative feedback that was left.
Example:
Trader A leaves a rating of -1, with a Reason/Comment that reads "Lousy trader. Sent me the wrong cards."
Trader B, who received negative feedback, posts a Rebuttal/Reply: "User received all cards in the trade proposal. He wanted specific editions, but did not state that in the trade proposal. I sent all M10 cards, but, after receiving the cards, Trader A indicated he wanted only M12 cards.
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