Most (maybe ‘all’ but I can’t prove that and most probably wasn’t all) of the slaves taken to the Americas and Middle East and Asia were already slaves to black empires within Africa.
These African empires - like the Benin Empire - sold their slaves to the Europeans and Asians.
The biggest slaver of all was Tipu Tip, from the Zanzibar Empire. Henry Louis Gates Jr’s research claims that Tipu Tip, as one man, had more slaves go through his books to Asia than the entire American slave trade in the same time period.
The slave trade within Africa with Africans enslaving Africans has a long history.
Even English Protestants sold captured English Catholics to the African slave trade during and after the English Civil War because the trade of slaves within Africa was a known profitable venture.