Slavery as a trading method was organized variously from one region to another along the coast. On that note, the European traders did business with the people of the Niger Delta and that led to the formation of the Royal Niger Company.
However, the captured slaves were taken to the hinterland markets where they were sold to the European traders. The trade was based on the barter system at the initial stage, which was later replaced by manilas, copper, iron, etc.
Note that the coastal business middlemen bought slaves from the interior markets and sold them to the Europeans hence, the demand for slaves was at a high rate.