The slave trade had sellers as well as buyers

In a NY Times oped, Ending the Slavery Blame-Game, Henry Louis Gates Jr. writes about the slave trade.

"While we are all familiar with the role played by the United States and the European colonial powers like Britain, France, Holland, Portugal and Spain, there is very little discussion of the role Africans themselves played. And that role, it turns out, was a considerable one, especially for the slave-trading kingdoms of western and central Africa. These included the Akan of the kingdom of Asante in what is now Ghana, the Fon of Dahomey (now Benin), the Mbundu of Ndongo in modern Angola and the Kongo of today’s Congo, among several others.
For centuries, Europeans in Africa kept close to their military and trading posts on the coast. Exploration of the interior, home to the bulk of Africans sold into bondage at the height of the slave trade, came only during the colonial conquests, which is why Henry Morton Stanley’s pursuit of Dr. David Livingstone in 1871 made for such compelling press: he was going where no (white) man had gone before.
How did slaves make it to these coastal forts? The historians John Thornton and Linda Heywood of Boston University estimate that 90 percent of those shipped to the New World were enslaved by Africans and then sold to European traders. The sad truth is that without complex business partnerships between African elites and European traders and commercial agents, the slave trade to the New World would have been impossible, at least on the scale it occurred."