A significant difference between historic slavery, practiced by many peoples in many historic time periods, and European/American Atlantic slave trade is that for the first time massive numbers of humans were exported to other parts of the world as captive slaves. This had a psychological effect, producing alienation and enmity, on the Africans who captured other Africans specifically for export. Another effect on Africans was the contribution to the dehumanization of potential and actual captives--a process heightened and accelerated in the Americas by American slave sellers and slave owners.
The major effect was a dramatic reduction in the male population, since male slaves bought a higher price than females. African tribes often raided each other with no intention other than that of providing captives to be sold into the slave market. Roughly two thirds of those sold into slavery were young males between the ages of fourteen and thirty five. The result was a tremendous gender imbalance, with substantially more females than males. As a result, women were forced to perform chores previously performed by men, and in some instances, polygamy became standard practice. Warfare between tribes and factions was exacerbated because slave traders often traded firearms for slaves. Conflict between areas frequently broke out which never would have occurred were it not for the introduction of guns by way of the slave trade.
Mainly, the slave trade made the coastal states much more powerful than they had been. The states on the coast were able to trade for European weapons and such and use them to weaken the states that were farther inland by taking so many of their people as slaves.