The correct answer is - The traders purchased the slaves from other African tribes.
The slavery in Africa was a common thing, as in most of the world, and the bigger and stronger tribes had lots of slaves from the tribes they've defeated earlier in a battle.
The Europeans needed a lot of labor force, and the stronger African tribes were ready to make a profit from their slaves, so they were selling them to the Europeans. The first ones that started buying slaves from Africa and move them to the New World were the Portuguese, and after that followed the other colonial powers.
