Slavery was part of Africa, just like it was part of Roman empire. The key unique thing about African slavery was that they were not the chattel slaves. They were provided with certain rights. Later when the Atlantic and Arab slave started the Africa became a major source of scales. Debt slavery, Military slavery, Slavery of war captives were forms of slavery in Africa. Slavery for Domestic and court purposes were most widespread. They were well treated.In larger African states slaves worked in the state administration, and could be an important royal official. Generally people from other ethnic groups were enslaved.
Slaves in Africa treated like family. Slaves not seen as personal property (chattel slaves). Certain rights were given just like indentured subjection like in the rest of the world. In Africa, slaves were used in different places like in domestic and court services. Slaves were also given training in military units to become military to fight against the enemies of his lord. In other words, Slavery was not harsh in Africa compared to America and other colonies where they were forced to suffer.