Karma takes many different thinkgs into account, so peer-to-peer rating doesn't have a heavy influence.
In the case of buying selling, the rating should only be based on the transaction, and you don't want it to be available to everyone (Only the buyer or seller should be able to rate each other) - therefore there would need to be a system in place to measure when a transaction takes place, then giving the buyer+seller an option to rate each other..
It would probably be less complex than the karma system, but it's still a big job for it to be effective. Then there are all the past sales which should be taken into account (which would be hard to recover the details of), (The good buyer/seller list is out of date and not all sales are recorded)
A lot to think about..for the timebeing, i'll base my opinion of a seller on their contribution to the website and karma.