We now have installed a popular ebaylike rating system for Vbulletin named "iTrader". You can use it to individually rate users after completed trades.
You can see a link in the postbit (where the postcount is written) and can also directly access it here. Abusing the iTrader system will lead to a suspension of your account without any prior warnings.
I have not had the chance yet to rate other users myself hence I have not done any recent trades. Your feedback is appreciated.