Slavery was common throughout Africa before, during, and after that time period. Slaves were provided as tribute to kings, captured in battle, or enslaved due to nonpayment of dept. Slaves were used for the full range of tasks from mining and agriculture to sex and domestic tasks. Customs regarding slaves varied widely throughout the continent.

The first slaves owned by Europeans were already enslaved and purchased from their owners. Demand for slaves on Europe and Africa led to an increase in inter-tribal conflict for the exclusive purpose of taking slaves for sale.

More info on slavery in Africa: http://www.latinamericanstudies.org/slavery/perbi.pdf

I would recommend Thomas Benjamin's "The Atlantic World". It's a recent and full overview of the Atlantic World.

Primary source examples suggest that there were status and class issues - so it was routine for families to sell off "lesser" members of their own family/extended family. Slavers were opportunistic and ruthless. Olaudah Equiano talks about being captured along with his sister when they were playing outside. http://www.bbc.co.uk/history/historic_figures/equiano_olaudah.shtml

There was a common contemporary belief that the majority or a large proportion of slaves captured and transported to the AW colonies were prisoners of war. Alexander Falconbridge, slave ship surgeon and later abolitionist writes about his surprise that the men and women he treats and sees do not possess the physical scars and injuries that he would expect from people who had recently been in a battle. Slavers preferred men over women, but happily sold women, even pregnant women, as well as children. There simply weren't enough people to keep up with the demand for slaves in the AW colonies.

Worth stating, of course, that there shouldn't be an expectation of 'African' denoting a shared identity or loyalty, just as there is no expectation of that in regard to 'European'.