Subject to Fourteenth Amendment protections, states are free to set their own standards with regard to how people are adjudicated mentally incompetent, and how the affairs of such persons are managed. It would seem that the legal standard for finding someone sufficiently mentally incompetent that they cannot be allowed to do things like make their purchasing decisions should be pretty similar to that required to disarm them on the basis of mental incompetence.
The 14th Amendment says that the state has the right to deprive a person of liberty or property so long as it is done with due process of law. So shouldn't that include the right to decide that a person convicted of a violent crime shouldn't be trusted with a firearm?