Every citizen has the right to produce their own firearms. His mistake was selling them.
Agreed on both points.
I've read that it's legal to sell a firearm that you previously produced for your own personal use, if the receiver has any & all federally required markings (manufacturer name & location, serial number, etc.). That's assuming the firearm itself (& the sale thereof) is otherwise compliant with federal/state/local laws & regulations - since I'm not a lawyer, I'm not planning to ever go that route...
Slight nit pic with your statement. Prohibited persons are not legally allowed to produce their own firearms.