Excerpts from Books and Wikipedia
"The island of Goree lies to the south of the Cape Verde peninsula, Senegal. By the mid-15th century, the Senegambia region was being transformed by the arrival of the Portuguese, and subsequently other Europeans. Once the islands indigenous Lebu people had been displaced, the town of Goree, founded by the Dutch in 1621, became a major slaving entrepot.
Many trading forts were established along the West African coast in the 17th century, with permission from local African rulers, to whom the Europeans paid tribute. Local rulers provided slaves, mainly captured in warfare, for the Europeans in exchange for cotton, copper, tin, iron, brandy, and glass trinkets. The slaves were then dispatched, in terrible conditions, to the sugar plantations of the New World, at a profit to unscrupulous traders of as much as 800%." [DK Timelines, p. 301]