Ok.
I learned in 1980s New Jersey that African tribes sold other African tribes into slavery because they had a custom:
When they had battles, they gained some of the losing men in arrangements similar to indentured servitude and they would trade them with other tribes they were trading with.
When the European traders came by, they believed that it was the same kind of arrangement; a temporary spoils of war slavery and that eventually they would be returned.
Now, this is what I learned in early 1980s New Jersey USA textbook – and we had an awful textbook that had blond haired and blue eyed cave men – with short hair and clean shaven faces.