American slave traders acquired African slaves from coastal towns in western Africa primarily from other Africans though Europeans would travel into the interior and capture slaves themselves, such as the Portuguese in Angola.

African leaders would often enslave people not in their own ethnic group since for many people, entering the slave trade was a way to protect your people. Guns were useful in a fight and Europeans were willing to sell them to Africans for slaves. The guns would then be used to acquire more slaves for more guns. If you refused to take part in the slave trade, chances were you would one day be enslaved by people that did. It should be noted that many Europeans would buy only slaves and so if you needed guns, you needed slaves. They would also extend credit to Africans buying guns, ensuring that they were obligated to supply slaves.

The point I'm trying to make is that it only took a few people to get the gun-slave-gun cycle started and once it did, it proved near impossible to get out of.