"Blacks enslaved blacks, make sure you get that straight."
Duh, no ----, Boaz Henry Gates. We covered that all throughout high school and college. It is no secret. However, American white men were by far the greatest beneficiary.
By and large, to the black African elite, selling fellow Africans to the white Europeans and Americans was just business, and those they sold were just property who had the misfortune of being in the wrong place at the wrong time. But to the European white, that 'property' was also considered fundamentally subhuman and debased, and thus the evil takes on a much more malicious and pernicious element.
So in summary: Black Africans who sold other Africans for profit: bad.
White Europeans who bought Africans while at the same time considering them inferior in every way (and who frequently used biblical references to substantiate their beliefs): much much worse.