In particular, did it change the rate of slave capture? I have seen it claimed that no more slaves were being captured during the slave trade than before. Or if that's wrong, did the increase in slave capture correspond with a decrease in people being killed in sackings or after having surrendered in war? Either of these being true would suggest that slaves being shipped to America would otherwise have been either slaves in Africa or dead.
Also of interest are cultural, economic, or geopolitical changes due to the slave trade. For bonus points, compare with Arab slave trade.
Africa already had an existing slave system where slaves were captured in wars. The trans-Atlantic slave trade increased the rate of these wars for two reasons. One was to get more slaves to trade. The second was the slaves were traded for guns, and if your rivals had guns and you didn’t, you would quickly be taken over by them. So all polities in Africa had a powerful incentive to engage in the slave trade, which required raids and wars to sustain.
An interesting thing about the slave trade is that at first, European goods weren’t particularly desirable. West African cultures were superior weavers, and European cloth was kind of like buying trendy cheap clothes at Forever 21. Sometimes they would unravel the cloth and re-weave it. Africans were also working iron using techniques that wouldn’t be perfected in Europe for several centuries; however, iron was relatively scarce and was expensive. European metal goods were used as cheap implements, like your Made in China stuff. Africans had a plentiful supply of male slaves to trade because the Arab slave trade preferred to buy female slaves.
At first, Europe didn’t have a military advantage either. African ships were highly adapted to coastal and river travel, and there are accounts of Africans winning naval battles. Portugal was only able to obtain its first colony, Angola, by making a military alliance with the king of Kongo.
Obviously, as Europe grew in influence and guns came into the picture, the dynamic shifted. So much trade gradually shifted local economies, and the slave raiding wars inflamed tensions between African polities. The slave trade was a self-reinforcing phenomenon so that the longer it went on, the more difficult it would have been for Africans to stop. And they didn’t have much incentive to stop, because there wasn’t a pan-African consciousness. They didn’t think of themselves as selling “other Africans” into slavery; they were selling rival nations. Polities that had a lot of control over the slave trade became increasingly powerful and saw the slave trade as a source of that power.
Also of note, African slavery was a much more open system of slavery — it wasn’t heritable and slaves could own other slaves and rise through the ranks of society. They didn’t necessarily grasp how horrible chattel slavery was in the Americas by comparison.