I am not famiilar with the Karma system, but if it's an open voting system where I can, say, give anyone I choose a rating... I see a definite problem. There is just no way to police that. You will have people rating as they please, not due to transactions.
Say I get into a heated debate with Bob on the boards here, and I prove him wrong after some spirited words.... what's to stop Bob from giving me some bad karma? Or.... let's face it.. we've had some jerks pass through here who just wanted to cause trouble.... "hey, let's give this guy a Negative cuz he didn't like me spamming"
I don't see how you could enforce it so that I can only vote if I actually bought or sold something to that person, and therein lies the problem. If you just don't get along with someone, what's to stop them from giving you a terrible rating. My feelings on the best car or better joystick, or whatever... have no bearing on how I conduct business.
I'm all for the honor system, but let's face it.. it rarely works. For every genuine rating you get from a transaction, you'll prolly have 2 or 3 false ratings from people. That's no way to decide who to do business with.
This is why I still say Heatware.com is the best way to go through this. It's third party, there is less chance of people giving you false ratings since they'd have to sign up themselves, etc etc, and you can talk to Heatware to get false ratings removed. It also works with any forum you read. I may have 2 post here, but have been selling on AnandTech for 5 years... Heatware can show you that. All you'd need is perhaps a sticky in B/S/T suggesting the use of Heatware.
Just my .02