The way this is framed is such a lie and that's why people get away with this argument.
Slavery before the slave trade was different. Most slaves were war captives and alot of them had the chance to eventually buy their way out of slavery. The slaves that were sold were either captives or stolen from other villages. It had nothing to do with HOW you looked.
Race as we know it didn't exist back then. The Africans didn't know wtf a black race was and neither did the Europeans, but now since race does exist as a social construct it's easy to say "Africans sold their family and friends" because we look at the race as a sort of kinship, but those people had no affiliation with the people they sold. It was bad, but it looks worse now because we view it as Blacks selling their family when in reality it was just people selling random people.