And to answer your question "what Muslims were enslaved besides by fellow Muslims" Most states along the Slave Coast and the Bight of Biafra controlled slave trading with Europeans on the coast. By and large, these were not Muslim states, but sold thousands of people to the "slave castles" that can be found on modern Ghana's coast or to European ships anchored off the mouth of the Niger Delta. Foreigners (many of them from the deep interior Sahelian regions where Islam was practiced since the 10th century) made up the vast majority of those sold on the coasts, often by non-Muslims, almost always to alleged "Christians".