The European slavers were buying slaves off of someone, and I would find it hard to believe that whoever was selling the slaves in Africa would ever accept low prices for their "goods".
Africa is a very big continent, and the slave trade happened in many places. It's difficult to generalize, so I'm just going to point to one example and a source.
In the Kingdom of Dahomey, for example, it was mostly the nobility who profited from the slave trade, with slaves coming from prisioners of war or criminals. One american, named Captain William Foster, for example, made a wage in 1860 that he could buy and bring slaves to the US even with the slave trade being illegal. He recorded his trip in his diary to the kingdom of Dahomey, and in there you can read about how he negotiated with the local prince for the aquisition of slaves.
Speaking of Dahomey, most of that kingdom nowadays is in the country of Benin, and the way history is taught in Benin is a very contentious matter in politics. The descendants of slave traders and nobility who profitted from the slave trade are currently part of the wealthy elite of the country. In a recent presidential election debate, for example, one candidate, Lionel Zinsou, accused the other, Patrice Talon, of being the descendant of slave traders. Talon won the election, by the way, and is the current president of Benin.
There were also agudas and tabom. They were Brazilians, mostly Black or mulattoes, who returned to Africa after being freed from slavery. Some of them dabbled in slave trade themselves. The most famous example is the chacha of Benin, Francisco Felix de Souza, probably one of the wealthiest slave traders of the time. There are statues of him in Ouidah, which was a big slave port, and some people nowadays protest these statues much like people do over Confederate generals or Apartheid or Eastern European communist leaders.
This elite, both aguda/tabom and native, who traded slaves were later used by the European colonists as middlemen with the rest of the natives (or at least this happened in French colonies, like Benin and Togo). Their descendants also became the first leaders of independent African countries in many cases, like Sylvanus Olympio, one of the first prime ministers of Togo. In some cases there was resentment against this elite for participating in both colonialism and slave trade, so many of the military coup d'etats that were seen after decolonization were about taking power from this part of society and giving it to the military, who most of the times had leaders from another ethnicity or class.
Sources:
Sylviane A. Diouf, Dreams of Africa in Alabama: The Slave Ship Clotilda and the Story of the Last Africans Brought to America (NY: Oxford University Press, 2007).
News piece about the agudas and the chacha (in Portuguese, sorry, I think you can find sources about him in French as well, but I can't read that language)
Washington post article about the Benin presidential election and legacy of slave trade