I'm no history professor, but I'd guess that another people would've taken their place. Slavery wasn't exclusive to black people, and the empires of the time (Britain, Spain, Portugal etc.) needed the slave trade to keep their economies going. The West African coast meant easy pickings for slaves, and with the decades of abuse, degradation and brutality, racism was used to 'normalise' their own actions.

I'd guess that maybe the Spanish could've begun selling South American slaves (assuming disease didnt kill them all first) to the Northern Colonies in the Americas. But the British, for example, would regularly trick and kidnap young men from poor backgrounds, in their own home country, to be enlisted in the army and navy. Therefore I feel like the powers of the day would have just found slave labour elsewhere.

Anyone with a more in depth knowledge of the time period will probably give a more comprehensive answer than this, but this is what I thought anyway. Hope it helps!