There's a very effective "trader" assessment system already available. Find someone who is not afraid to speak their mind, doesn't change their stance on manufacturers depending on what he/she is using/selling at the time, is generally a little grumpy and desn't mind upsetting people.
I have been to Terry (tsauto) in Stourbridge a number of times and we do not even mention the cost until after the work is done and I am about to leave, let alone ask for payment up front etc.
I believe I could work with Trikey and Brian plus a few other here on a similar basis with absolute trust.
The probem with a trader rating system is that I don't think I would believe the jugement of all the members!
p.s. also, if you cannot have a natter and a cuppa for 1/2 hour before the chap even looks at the car, he's not worth bothering with.