It’s fairly common to see someone mention that Africans sold each other off into slavery at the height of those days.
What’s not very common, that I have seen, is for some original/old (preferably reliable) source given to go along with the mention.
Selling captured enemies was part and parcel of war throughout human history, and a way to weaken the enemy without having to kill all of them.
What kind of documentation would you like from illiterate societies?