The transatlantic slave trade had very different impacts on North America and Africa. Let us briefly discuss the most important effects.
One of the major effects of the slave trade on North America was very positive for all of the non-slave settlers of that region. It is fair to say that the prosperity of North America was built in significant part on the backs of slaves. This is clearly true of the South. In that region, the economy was dominated by large plantations whose labor force was provided by the African slave trade. Even the North, though, was helped greatly by the slave trade. Ships from northern ports participated in the slave trade. Companies from the North did things like insuring slaves and making things to sell to the plantation owners. In these ways, the economy of North America (at least the parts that became the US) owed much to slaves.
The other effect on North America was less positive. The slave trade led to the creation of a society with both whites and blacks, free and slave. This led to some very important conflicts. The greatest of these was, of course, the Civil War.
The slave trade devastated parts of Africa. African coastal states raided inland to take slaves to sell. This deprived inland states of many of their most important members. When this happened, the states were damaged in terms of their economies and their cultures. The coastal states got rich selling the slaves, but the inland states were badly hurt by the loss of so many adults in the primes of their lives.