[Apache6]

The majority of Africans enslaved were captured by African tribal chieftains. Originally they would have been what we would call POWs as well as the 'spoils of war' from conquering neighboring tribes.

Arabs and Europeans both traded goods (either manufactured goods (knives/axes/pots/spear points/firearms/shot and powder) or the classic Rum was used as trade goods. They usually pulled into port and offloaded the trade goods and unloaded the slaves who were being held in preparation for the sale.

After the chieftains learned that there was a profitable market for their captives, it was not uncommon for "military" expeditions to be launched primarily for the purpose of capturing slaves for trade.

Sometimes Arabs or black Muslims would undertake expeditions into Africa with specific intent to capture slaves themselves. The North African Corsairs were notorious for raiding the Mediteranean Europe (and rarely the Atlantic Coast of France and England/Ireland). They were after booty, specifically including women and child slaves, though they would take ships and anything else of value as well.

Unfortunately we have recent real world examples to use as well. Over the last 20+/- years, in Darfur (South Sudan) , "Muslim militias" have enslaved thousands of people (mostly women and children) as part of their ethnic cleansing of certain regions. Likewise ISIS admits to taking slaves in their operations against Christian, Yazdi, and Shia populations.

ISIS is clearly using the ideal of taking women as slaves as a recruiting tool, unfortunately it's an effective one appealing to the large numbers of young men in sexually repressed Sunni cultures. The fact that older/wealthy men often have more than one wife, including those MUCH younger than them, contributes to dissatisfaction among the young men.