But you see, for trade you need partners. You can't trade with the enemy - and please stop pointing out examples from the history. Those are the exceptions that prove the rule. Remember the mutual dislike the Zuul share with other species? That is all they are willing to share, really. So trade is something they do not want - probably because it is beneficiary to both sides. They thought of slavery instead. Dedicated ships for harvesting slaves make up for missing trade ships anytime. Slaves can also be taken to Zuul worlds and become a welcome addition to the factories.