My contribution to the discussion of the peculiar institution is to observe that it was not peculiar. The term refers to a concept from Roman law, that is, peculium , which was the right of a slave to own property. The slave may have been the property of some Roman master, but he (probably not she) had the right to some property in his own name. It was possible for a slave to own other slaves.
The point, as applied to U.S. history, is that slaves in North America had no right of peculium. Roman slavery may have been peculiar, but North American slavery never was. I suspect the planters called it the peculiar institution as an implicit argument in favor of the practice by harkening back to supposed historical antecedents from antiquity. In that, as in so much else, the planters were deluded.