A slaver is a human trafficker who buys and sells slaves. Said slaves are often foreign prisoners of war, but could also be criminals, unfortunates unable to pay their debts or people kidnapped from their homeland.
The biggest slavers of the Mediterranean Sea were the Cilician pirates, who were known as the hijackers and kidnappers of the ancient world. They would kidnap people near the coast and sell them to the Romans. The Romans mostly depended on the pirates for their slaves but they were also a problem to Rome and it's people.