Both are true, slaves would be taken during war by african chiefs and then sold to europeans

But It is important to recognize

1. European chattel slavery and the slavery in Africa were different

2. The atlantic slave trade adds an major economic incentive for chiefs to raid other tribes for slaves to sell

https://www.reddit.com/r/AskHistorians/comments/82lcvr/did_precolonial_african_societies_practice_slavery/

By /u/medium_well_soyuz_1