By the time of the American Civil War, West Africa had been irrelevant to American slavery for several decades, and vice versa. Following the abolition of the slave trade in the British Empire in 1807, the Royal Navy established the West Africa Squadron with the aim of interdicting slave ships continuing to participate in the trade. The US itself banned the import of slaves in 1807, taking effect in 1808.

By the time of the American Civil War, the slave trade within America was almost entirely domestic. That is to say that slaves were essentially 'bred' in the more populous 'Upper South' and then transported to the more Southern states where their labour was in constant demand.

I recommend Deyle's The Irony of Liberty: Origins of the Domestic Slave Trade.