The "slave trade" as defined in contemporary society began in West Africa in 1441 by the Portugese captain Antam Goncalvez. The explosion in the trade took place when the Portugese, English and Spanish discovered a ready market for slaves in the African country of Ghana. In Ghana, there was an excess of locally-mined gold metal and a lack of manual labor to tend the farms of affluent Ghana families. European sailors traded captives to the black Ghanans for gold and "legitimate trade".
It's no coincidence that on the west coast of Africa the "Gold Coast" and the "Slave Coast" are very closely approximated.
For the definitive contemporary work on this subject, refer to John Reader's text "Africa, A Biography of the Continent" originally published in 1997 in Great Britain by Hamish Hamilton and in the USA by Knopf.