Naw, it's for anybody - buyers & sellers alike.
You're welcome to post whatever ya want in the forum itself, but if you want to make absolutely sure the guy you leave feedback for gets credit (gets the feedback added to his rating) make sure you post 'em directly in the main FEEDBACK
thread. That's where most of the numbers are harvested from.
BTW, I just added a "hot" feedback icon for you in the main thread (click on it).