Slaves were largely taken from conquered enemies, there was not an industry of taking and gathering slaves. African economies were instead more feudal and communalistic, rather than slave-based. It wasn't until the arrival of the Europeans that slaves became as important as they did for gaining technology and avoiding invasion by the Europeans, and thus the slave trade proper was created. There existed proto-slave trade formations that grew vastly.

I think you're getting a bit hooked on how I'm wording things. Am I denying the existence of anyone buying a slave ever before the Europeans arrived? No. Am I denying that there was a real slave trade, an economy of slavery, before the Europeans arrived? Yes.