Most traders have a Whitelist and a Blacklist in their trade threads to show to other people who they think are bad traders for whatever reasons they have for them. Whether it be a bad trade, or abuse over the wifi, or something like that. I don't think this is really necessary, as someone might have a good experience with someone, while someone else could have a bad one.
If anything, I think there should be a blacklist of People who have been proven to hack Pokemon or something, which are decided by the Trade Corner moderators' (TwilightBlade, Aurafire) judgement.