Slavery was common along the Upper Congo River .
In the second half of the 18th century the region became a major source of slaves for the Atlantic Slave Trade , when high slave prices on the coast made long - distance slave trading profitable .
Slavery was common along the Upper Congo River , and in the second half of the 18th century the region became a major source of slaves for the Atlantic Slave Trade , when high slave prices on the coast made long - distance slave trading profitable .