Two things stopped Dahomey. The American Civil War and the European Kings decision to invade and colonize Africa.
Once the American Civil War started Dahomey had no other place to sell people. That and the English and American blockade of West Africa ended Dahomey's reign of terror and they could buy no more arms from the Europeans because the Europeans were planning on invading and didn’t want to face well armed opposition.
Interesting re the US Civil War
Weren’t Cuba and Brazil bigger markets?