A question, then. To whom were enslaved persons being sold? Within Africa, it was expensive in terms of time and resources. No plantations, no national- or international-scale socioeconomic reinforcing the institution.

The biggest slave traders were from Africa

That, my friend, was not on an industrial scale until the Europeans created a transnational market for it.

Yes, slavery is a part of many histories. Prisoners of conflicts, poverty, inter-family disputes, etc. led to people being enslaved. It was also normal, to the point of socially reinforced, to free slaves. This was known as manumission.

The transatlantic trafficking in humans, and the perpetual enslavement of their descendents was on a scale unlike anything before. They were the “biggest” slave traders only if/when you neglect all the descendents born into bondage. That is a virtually unique stain on human history.

Further, in virtually every society on earth, it was not “race-based” bondage. It was debt-bondage or honour-bondage as an alternative to being killed in battle. There was a path to freedom, and a comparatively short one compared to institutionalized slavery.