In compiling those to whom Africans were (and maybe still are) sold by other Africans, don't forget the originals where sub-Saharan Africa is concerned: the Arabs. Over fourteen centuries, 18 million Africans were sold at the markets in Fez, Cairo and, yes, Mecca. And they were a minority of those sold and taken: up to 120 million died in the Sahara on the overland trek. All of this can be read in John Azumah Adembillah's book, The Legacy of Arab Islam in Africa.

Keep in mind also that the Arabs castrated most of the men they took. Read about the Zanj uprisings in the Tigris-Euphrates delta where the Arabs once grew sugarcane with largely African slave labor.