Slaves in those days were seen as like cash in the bank today. Some of the blacks that owned black slaves were in fact the mix race offspring of white plantation owners and slave women. Some of these white plantation owners left (in their wills) some of their slaves for their mixed race children as a way of taking care of them.
Exactly. In addition many of the black "slave owners" were freed men who bought their own family members. In some Southern states slaves who were freed after a certain year were forced to leave the state or risk re-enslavement. Those already freed were grandfathered out of that rule, so that freed men, in order to keep ties to their family, had to buy them.
Of course those who will justify slavery will use the fact that a tiny % of blacks owned slaves for commercial purposes to justify this odious system.
Another little secret is that MANY IRISH were SLAVES too!