I remember when I was much younger, before Infinity even came out, I was under the impression that the Jjaro were a race of evil slavers like the Pfhor, because of the existence of "client races" of them. (The earliest draft of a sequel to M2 that I wrote back then, which eventually morphed almost into the almost-completely-different plot to Eternal, involved the Pfhor's "old allies" the Jjaro returning).
In my later interpretation, I took "client" to be a general term of subservience. The Jjaro were as gods to their client races, which is not unlike the master-slave relation, though framed much more benevolently. The Pfhor were more traditional "mortal" masters of their client races. In either case, the Jjaro or the Pfhor are the ones in power calling all the shots on whose will their clients' continued existence and entire lives are dependent. One is just seen as deserving of that power, being the creator of their clients, like a parent to a child; the other is seen as undeserving of it, as a conqueror or, as you suggest, perhaps merely owner from trade.
One thing I find difficult to reconcile with the idea that the Pfhor have trading partners is that Tfear explicitly states that Pfhor policy is to destroy that which they cannot conquer, referring here to the use of the trih xeem to put down the S'pht revolt. That seems incompatible with the Pfhor having anything like peers: they meet someone, try to conquer them, and if it fails, blow up their stars and destroy them.
Then again, Durandal thought that the Pfhor had retired the trih xeem, so maybe the Pfhor have lately been trying a lighter touch, and only pulled out the trih xeem against the S'pht because of the severity of threat that their uprising posed.