Well, you have to look at the time when Europeans discovered the Americas and triangular trade system at the time. Let me explain:

It is the late 16th century, and the Spanish Empire has began to explore much of Latin America and South America. They discover that it is ripe for the taking. They first subjugate the surrounding areas, killing or enslaving Native Americans by the scores. In the Americas, the Spanish spied the extraction of precious metals first. They also realize that the conditions for commercial farming is perfect in the new areas. The Spanish Empire utilized the encomienda system, which allowed for them to draw upon forced turns of labor from the Indians to work their mines and plantations. The combination of Old World germs and harsh Spanish treatment led to a rapid depopulation of Native Americans. This required a new labor force: enslaved Africans. Africans were exposed to Old World diseases through the trade networks in the Old World, so they were less likely to die from those diseases.

Now, understand that there is a myth and stigma of discussing slavery. Some people believe that Africans were happily living in their villages and the white Europeans captured them and enslaved them and sent them across the Middle Passage. No. The African societies willingly participated in the slave trade, although later on in the 18th and 19th century some African kingdoms realize the moral wrong of what they had done. But I digress.

Slavery, for most people, seems to have this perception of existing around the time of European colonization. That is wrong- slavery was (and hopefully will remain a thing of the past) one of humanity's oldest institutions. Slavery predated European society and even African society. African's experienced a different kind of slavery in Africa- the Muslim states/entities viewed slavery as a a stepping stone in the conversion process of Islam. These people were considered family and their posterity would be considered free. As you most likely know, this was not the case in European systems.

There were internal trading networks prior to routine European contact. But as Western Europe grew stronger throughout the 1600's and 1700's, so did their influence. Western European societies established factories (coastal forts w/ merchants) that tapped into networks. Factories like the El Mina were the first of many factories established on the African coast. This allowed for Europeans to start trading refined goods- (gunpowder and guns).

European powers were limited by their factories. They were as only as powerful as the range of their muskets. But Europeans bribed and gave gifts to local rulers to start participating within the African slave trade. African society was relatively unstable with various warring kingdoms and tribes. This gave importance to a warrior-class. African conflicts tended to result in the victor to enslave the losing party. European guns, were a vast improvement over the native African weapons. So- the need for a hardy labor force in the Americas and the militant culture of Africa coincided to form a symbiotic relation between Europe and Africa. Europeans would supply slave traders with weapons, which would increase their power and worsen cyclic violence in Africa. In return, Africa supplied slaves.

So, here is the answer that you are looking for:

White Europeans exploited preexisting conflicts in West Africa to gain slaves. It was the exception that whites journeyed into Africa to snatch Africans, but rather a victorious clans selling of prisoners of war to willing European merchants. I can follow up with you more if you can narrow you questions down a little more.

Source: AP World History: World Civilizations Chapter 19 - Early Latin America Chapter 20- Africa in the Age of Atlantic Slave Trade