excerpt from slave trader John Newton's book "Thoughts upon the african slave trade":

"The african law punishes some species of theft and slavery; and in cases of adultery, both the woman and the man who offends with her are liable to be sold for slaves. I believe many of the slaves puchases in Sherbo, and probably upon the whole Windward coast, are convicts, who have forfeited their liberty, by breaking the laws of their country. I judge, the principal source of the slave trade, is the wars which prevail among the natives.

another slaver named Captain Theophilus Conneu wrote "how to free black becomes a slave":

"In africa, where coin is not known, the slave is made a substitute for this commodity. Therefore, if a man wants to purchase a wife, he pays the amount in slaves; another wishes to purchase a quantity in cattle, he tenders the payment in slaves. Fields of cassava, rice, or yams are paid in slaves. The African court also taxes all forfeiters and pecuniary penalties in slaves."

you can read the whole piece "African roots: slavery was widespread on the African continent long before Europeans appeared - and, indeed, is still practiced there" here:

http://www.findarticles.com/p/articles/mi_m1282/is_n4_v49/ai_19208900