Slave traders don't sound like an exceedingly rapidly replenishing food supply. Slave traders are businessmen. If there's too little profit compared to the expenses, they disappear. Getting killed is the most notorious expense. Of course the slave traders can just stay back and send in mercenaries and other henchmen, but if those keep getting killed, new mercenaries will be hard to hire or they will demand absurd compensation for the dangerous work. In which case no more men will appear.