Africa is who sold the Europeans the slaves. This was 'standard practice' worldwide. Does not make it at all okay in any sense of the word, EVER. 

But this history is why Human Trafficking is taken so seriously today, as it should be. It is real. It exists. And it happens in America. 
---
'Why were African merchants willing to sell slaves to Europeans? Were they “selling their own people”?'
[Snips] 
* The majority of people sold to Europeans as slaves were prisoners of war, thus outsiders, and this was an acceptable fate for prisoners throughout most of the world
* Africans did not identify on racial or national lines, but on very local levels and along lineage lines
* They were not selling “their own,” any more than Europeans were purchasing “their own.” No
“African” or “Black” identity in Africa during the slave trade
http://africa.unc.edu/outreach/ASA/slavery_and_trade.pdf
---
What is Modern Slavery? - Department of State  
https://www.state.gov/j/tip/what/