The slave trade had many effects on Africa. Most of them were, of course, negative, though we can argue that the slave trade was beneficial for some African states in the short term.
One negative about the slave trade was that it tended to increase the amount of war that occurred in West Africa. The reason for this is that European (and American) slave traders did not simply go out into the African countryside and kidnap their own slaves. Instead, they bought slaves from the coastal kingdoms. Those kingdoms generally got slaves to sell through war and through raids against inland tribes. Because the slave traders wanted more slaves, the coastal kingdoms were encouraged to wage more wars and conduct more raids against their neighbors. In addition, those kingdoms were provided with things like guns in exchange for slaves. This helped those kingdoms have a greater capacity for waging war.
This brings us to the one (short-term) benefit of the slave trade: it initially helped the coastal kingdoms. Those kingdoms became richer and more powerful because they were able to get guns, money, and other things in exchange for the slaves.
However, even these kingdoms were hurt in the long term. This is because the slave trade hurt all of West Africa. First, the slave trade took away millions of Africans (men more than women) in the prime of their lives. This badly disrupted both the cultures and the economies of the African nations. Because they were disrupted, they were less able to progress. The link below argues that the slave trade made it harder for Africa to enjoy an agrarian revolution and, in turn, an industrial revolution. This is because the men and women who could have helped make these revolutions were being taken into slavery. Because the African nations did not develop economically and because their societies were weakened, they were unable to effectively resist the Europeans when the Europeans started to colonize Africa.
Thus, we can say that Africa was badly harmed by the slave trade. The trade made war more common, harmed the economies and societies of the nations from which the slaves came, and eventually made it easier for Africa to be colonized by the Europeans.