Slave trade: Did Africans sell themselves?

An argument is being made to suggest that Africans themselves participated in the slave trade by catching and selling off its own people. This argument presupposes that without the help of Africans themselves, the Europeans would not have engaged or succeeded in the slave trade. Is that the case? Is the argument that without the help of fellow Africans, the slave trade would never have happened? If that is the case, then a similar case could be made for colonization in general. That without the help of fellow Africans, Africa would never have been colonized. Now that sounds too simplistic, doesn’t it? 😀 The story is just not that simple. Did Africans who participated in the trading of slaves did so freely or under pressure? Or another question is, would the slave trade still have happened anyway, without the help of fellow African traders?