Slavery developped with the emergence of agriculture and the domestication of animals and herding in and after the Magdalenian. It existed on all continents and in all civilizations. With the development of the Mediterranean as an important commercial, social, and political hub covering what was to become Europe, what was the Middle East with the Fertile Crescent, and what was Northern Africa and teh Sahara. These civilizations had their own native slaves with captured prisoners in wars and to expunge some debts, or other problems of the type. The Black Pharoahs were military personnel originally in Egypt who captured power. They were originally slaves. Black Africa was an important source of slaves with two main routes. One across the Sahara to the Mediterranean Basin, and the other from Eastern Africa across Lybia or along the coast of the Indian Ocean. But slavery was not limited to Black people. The famous Chinese Admiral Heng Zhe was a slave, had been castrated, and was the master of sea voyages and discovery all around the Indian Ocean. He was not Black, and he was a Muslim from what is today China. We are at the beginning of the 15th century some more than 50 years before Columbus and with ships that were at least five times as big as Columbus' ships. Check my book on the subject of the Indian Ocean and Container Maritime Commerce. There is a first part on the subject. You'll find it in all Kindle stores at Amazon.

First of all one tribe sold another tribe. So selling the other tribe was a guarantnee their own tribe will not end up in slavery. Second they probably got the same thing as from Europeans, i.e. trtinkets or nothing at all. And third, they were able to keep a position of power thanks to the support of the slave-buyers. And at times one tribe that were the victims of this slave commerce could or managed to take over and they became the tribe that sold the other tribe (the one that had been on top for some time) into slavery: this happened several times in the Mali empire long before the Europeans got into the picture. But the real question was what happened to the slaves. In the west they were transported to distant countries where they had to work for a pittance, including the Northern Africas countries long before they became Muslims. But in the east many had to be young males around 12 because they were very systematically castrated on their way to the mediterranean basin, very often flush with the abdomen. We estimate that half of them died of complications, blood loss, or infections.