I offer you this slave, he will do anything you need,for a great price.
Great! Deal. Bring me more and I'll give you more of what you ask.
I want one.
In 1441 when the Europeans were looking for a trade route to India and China but landed in Africa instead, where slavery already existed between Africans .
Africans would enslave criminals, debtors, or captives in war. when Portuguese got there they established empires. Allowing them to discover slaves and enslave some of the Africans taking them back to Europe.
Once Europe discovered the "new world" they started trading slaves with the southern states of North America. By the end of the 15th century, they made slavery purely an economic transaction, starting large slave trade within and beyond Africa.