I also think that the eBay approach is the right one. Have in mind that eBay uses it after lots of years of experience so it has been proven to be helpful. And I also think that having such values will actually make people to start looking deeper before making a deal rather than the opposite. There are a lot of newcomers that don't really know what (and why) to investigate about a user and I don't think that it is really needed for somebody to be scammed in order to learn a lesson.
Actually I think its a great idea, but I doubt it will be implemented. If the plugin/hack already exist then maybe, but if it has to be created and implemented I really doubt it. The itrader system could really use some improvements.
My one and only iTrader was a red return, I paid over $150 for the privilege. I did check the iTrader of the chap I was dealing with, but I forgot to factor in that they were mostly for tiny amounts vs. the bigger sum I paid. Oh well, you live and learn. I wear my -1 with pride at the moment, because anyone who knows me, knows I wouldn't do the dirty on them. Separating iTrader into +ve and -ve will clutter things up more. It should stay as it is imho.