I meant to start this list a while back as a partial remedy for all the scammer accusations flying around. While the introduction of secure trade will soon make this list obsolete, I hope folks continue to favor those listed here in future trades. They've proven themselves to be decent players:
Madbull/Madball (sorry, can't remember your exact in-game name!)
Silentreaper -- though some balk at his prices
These are the folks with whom I've had direct experience. Please add to the list.
p.s. if another list like this exists, I apologize. I was unable to find it.