Were the slavers forced at gunpoint to purchase humans?
That's a rather stupid question. Why would they be forced at gunpoint? Collecting and trading captured Africans from other African tribes was their trade, how they made their money. No white man was going to go marching through the jungles of Africa back then to hunt down blacks on their own. That's why they relied on Africans to capture other Africans. And slaves sold in the slave trade didn't all come from just Africa.
Slavery was common in Africa for many centuries before the Atlantic slave trade. Slaves were kept locally, but a great many were sold to Muslims on the Mediterranean and in the middle east.
Slavery was a step up from simply killing all adult males who lost battles in warfare. Of course, a great many people were killed and eaten.
Women were virtually always treated as a commodity by tribes.
Ask George Washington.