Answer:
The slavery in Africa was nothing weird, as most of the tribes had slaves, which were usually people that they have captured in battles that they won with other tribes.
When the Portuguese and later the other European colonial powers came to Africa and started purchasing the slaves from the local tribes, the slave trade changed. The local tribes were either paid, or the slaves were exchanged for weapons. That led to a fear in the rival tribes, so they started capturing more and more people so that they can get weapons too. That led to much more local battles in order to get slaves, which of course benefited the Europeans as they needed a lot of labo force for their plantations in the New World.
Explanation: