Is African slavery partly the fault of Africans?

I heard something very interesting from a friend some time ago, and I just remembered it today. His name is Dave, and he told me that African slavery was partly the fault of Africans because they sold off tribes they conquered to European settlers. He said he didn't remember where he heard that; he just remembers that he heard it. I don't find it 100% hard to believe, so does anyone have any proof proving or disproving what Dave said? I'm really curious.