In different time periods, things like currency and trade are more important than others. People could fall into slavery due to crimes and be punished by their own people. In the europeans, rather than steal your own men that man stealing is another term that they use to use for slavery. And it's a key reason why even people who had nothing to do with slavery ostensibly were impossible to disentangle from the slave trade.
The Atlantic Slave Trade mixes centuries of human bondage with violence, economics, commerce, geo-political competition, liberty, morality, injustice, revolution, tragedy and bloody reckonings. That sounds like a lot, yet this show merely scratches the surface of this enormous subject.