I think it was different in most places. Some were captives of war, others were folks who had commited crime. In my part of Nigeria(Igbo land), there was a group(the Aros) that dealt with slave trading, they'd go inland and get the slaves and take them to the port of Calabar, from where they'd sail on the Atlantic. The trails are still there, though somewhat covered over the years, but most local villagers know the path they took.