Between 1400 and 1900, the African continent experienced four sizeable slave trades (1).
1 Trans-Atlantic slave trade where slaves were shipped from West Africa, West Central Africa, and Eastern Africa to he European colonies in the New World.

2. The trans-Saharan

3. Red Sea - slaves were taken from inland of the Red Sea and shipped to the Middle East and India.

4. Indian Ocean slave trades –In the Indian Ocean slave trade, slaves were taken from Eastern Africa and shipped either to the Middle East, India, or the plantation islands in the Indian Ocean.

In total, close to 20 million slaves were taken from the continent (Nunn 2008). According to the best estimates, by 1800 Africa’s population was half of what it would have been, had the slave trades not occurred (Manning 1990).
during the slave trades, individuals frequently turned on one another, kidnapping, tricking, and selling each other into slavery. The existing historical evidence indicates that these forms of betrayal were common. (2)
The most common manner in which slaves were taken was through villages or states raiding one another (Northrup 1978; Lovejoy 1994). In many African societies there were no prisons, so people who were captured were sold." (3)

The Island of Zanzibar was the center of the Afro-Indian slave trade(4) After being gathered together and shipped from Zanazibar the slaves were shipped to the new location. Three out of four slaves died before they reached the market where they were to be sold. The causes were hunger, illness or exhaustion after long journeys. Once in the new location, the males were typically castrated. There, they worked as field workers, teachers or harem guards, which is why the castration of male slaves was common practice. (5)
STILL WRITING - THIS IS NOT FINISHED

References
(1) https://cepr.org/voxeu/columns/understanding-long-run-effects-africas-slave-trades
(2) https://cepr.org/voxeu/columns/understanding-long-run-effects-africas-slave-trades (the table can be found here)
(3) https://scholar.harvard.edu/files/nunn/files/empirical_slavery.pdf
(4) https://www.dw.com/en/east-africas-forgotten-slave-trade/a-50126759
(5) https://www.dw.com/en/east-africas-forgotten-slave-trade/a-50126759

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Highly educative. Thanks for sharing this.