In 1807 Britain outlawed slavery. In 1820 the king of the African kingdom of Ashanti inquired why the Christians did not want to trade slaves with him anymore, since they worshipped the same god as the Muslims and the Muslims were continuing the trade like before.
The civil rights movement of the 1960's have left many people with the belief that the slave trade was exclusively a European/USA phenomenon and only evil white people were to blame for it. This is a simplistic scenario that hardly reflects the facts.
Thousands of records of transactions are available on a CDROM prepared by Harvard University and several comprehensive books have been published recently on the origins of modern slavery (namely, Hugh Thomas' The Slave Trade and Robin Blackburn's The Making Of New World Slavery) that shed new light on centuries of slave trading.
What these records show is that the modern slave trade flourished in the early middle ages, as early as 869, especially between Muslim traders and western African kingdoms. For moralists, the most important aspect of that trade should be that Muslims were selling goods to the African kingdoms and the African kingdoms were paying with their own people.
Read the whole thing. It's a simplified version, but it's how slavery worked in Africa.