Well then you are not explaining it very well. If you sell person.slate.com to someone when slate.com is already sold to someone else, that means who ever owns

Well then you are not explaining it very well. If you sell person.slate.com to someone when slate.com is already sold to someone else, that means who ever owns slate.com cannot have their own person.slate.com subdomain. Tell me how DNS is gonna figure that out from a string of text URL? What you propose cannot be done with out taking something from the people who already own a particular domain.