I found this quote on this site:
Where did the supply of slaves come from? First, the Portuguese themselves kidnapped some Africans. But the bulk of the supply came from the Nigerians. These Nigerian middlemen moved to the interior where they captured other Nigerians who belonged to other communities. The middlemen also purchased many of the slaves from the people in the interior . . . . Many Nigerian middlemen began to depend totally on the slave trade and neglected every other business and occupation. The result was that when the trade was abolished [by England in 1807] these Nigerians began to protest. As years went by and the trade collapsed such Nigerians lost their sources of income and became impoverished. 4
This quote comes from this text:
4 Michael Omolewa, Certificate History of Nigeria (Lagos, Nigeria: Longman Group, 1991), 96–103, cited in Dana Lindaman and Kyle Ward, History Lessons: How Textbooks around the World Portray U.S. History (New York: New Press, 2004), 79-83.