You have to also understand that many neighbors hate each other far far more than a distant enemies regardless of evilness scale. Iran vs Saudi Arabia, China vs Japan, S Korea vs N Korea, India vs Pakistan, Scotland vs England, Alabama vs New York :). Many people in these groups will happily sale people on other side as slave to green aliens in exchange for wealth and dominance.

After you understand this, its not hard to imagine that white slave traders would naturally leverage African tribes at constant wars with each other to capture other party's members for money. What's better than killing your enemy? Make them disappear permanently while becoming rich and powerful!

I also doubt if African tribes actually understood this as part of war against their own race. The white salve traders would have surely treated their helpers very nicely and respectfully while introducing themselves as service providers to take their enemies away in exchange of money. People in Africa would at that time have little means to know what these white slave owners in Western world really thought about them and how they treated them because of their skin color.

So the slave traders systematically exploited same trick that colonials used to dominate vast populations with extremely small number of their own people: identify factions, take sides, wait till one faction gets weakened in economic and military strength from wars, load them up with debts and fear, and finally submit them to obedience. The classic divide and conquer strategy works every time through out the history in all kind of different problems.