Certain tribes waged war just to capture slaves -- for guns and random goods. They weren't their enemies -- but yes -- often times "strangers."
And more than a few chiefs got played by Europeans after supplying them slaves by having their own family members and members of tribe tricked into slavery.
In some cases parents were selling their own children and kin.
Children were snatched while working in the fields, walking on the outskirts of town, or innocently playing outside away from their parents' view. So that communities could make ends meet during times of famine, families sometimes sold their children into slavery. Many children also found themselves as pawns or bargaining chips, sold into slavery to repay debts or crimes committed by their parents or relatives. Some parents sold children who were in poor health, required special needs, or perceived as evil spirits.Source
The Asante allowed the Portuguese to build the
Elmina Castle in 1482 to promote the trade. Yes, they wanted to trade other things/goods - but they also wanted to trade/sale slaves - and they allowed it to become their number one product.
Same for when King Haffon of Whydah authorized the "Fort of St John the Baptist of Ouidah" in 1721.
To other willing collaborators:
The Story of Africa| BBC World Service
“…I must own, to the shame of my own countrymen, that I was first kidnapped and betrayed by some of my own complexion, who were the first cause of my exile and slavery…If there were no buyers there would be no sellers.”
—
African abolitionist Ottobah Cugoano (1757-1791)
“I want to apologize for the role my ancestors played in the slave trade….I knew one day I wanted to come to this land and ask forgiveness of my black brothers and sisters. I wanted to cross the ocean to see the land where my ancestors suffered.”
—
King Kpoto-Zounme Hakpon III of Benin to a black audience in Alabama, 2013
“We cannot continue to blame the white men, as Africans, particularly the traditional rulers, are not blameless….In view of the fact that the Americans and Europe have accepted the cruelty of their roles and have forcefully apologised, it would be logical, reasonable and humbling if African traditional rulers…[can] accept blame and formally apologise to the descendants of the victims of their collaborative and exploitative slave trade.”—
Civil Rights Congress of Nigeria, 2009
“I believe there is a great psychic shadow over Africa, and it has much to do with our guilt and denial of our role in the slave trade. We too are blameworthy in what was essentially one of the most heinous crimes in human history.”
—
Former Ghanaian diplomat to the UN Kofi Awoonor, 1994
An African country reckons with its history of selling slaves