I have always doubted the historical consensus that Africans sold their own people into slavery. The whole point of slavery is that you’re forced to work without pay. If whites were powerful enough to force millions of blacks to work without pay, they would have been powerful enough to take those workers from black countries without having to pay for them in the first place.
Also, if they were going to pay for slaves, then why go all the way to Africa to get them? The whole point of going to a less technologically advanced region to get slaves is that you can take the slaves by force.
Lastly, if you believe in HBD (which many people dismiss as racist pseudoscience), the average white American is about 15 IQ points higher than the average black American and that gap would have perhaps been 20 points before white genes entered the U.S. black gene pool in large numbers. A 20 point IQ gap (if it reflects a genuine gap in intelligence) is so large that whites would have been dominant enough to simply help themselves to whatever resources they wanted in sub-Saharan Africa (including human ones) without having to pay anyone anything. In fact if whites really had to pay for slaves, it calls HBD into question because it implies a business deal among equal races, not an advanced race enslaving a more primitive one.
Ironically, Professor Black Truth agrees that whites stole (rather than paid for) black slaves but he uses it to argue their moral inferiority, not their cognitive superiority: