For their descendants to show damages, it seems to me, they must show that, but for the removal of their ancestors from Ghana (or wherever), they would be....what?
As wealthy as the average Ghanian?
As safe as the average inhabitant of Sierra Leone?
As healthy as the average resident of Ivory Coast?
Have the life expectancy of Togo?
A trial will show that the descendants of American slaves have benefited, not been damaged, by the experience of their ancestors.
Now, this does not by any means justify the injury inflicted on their ancestors. But the ancestors cannot recover, because they are dead. A tort plaintiff has to be damaged personally, AFAIK.