Thoughts on private gun sales bans and registration
Here are my own thoughts. A ban on private sales will be unenforceable in the near future, since guns being found will have been initially sold before its effective date, and thus a lawful private transfer before the ban went into effect cannot be ruled out.
But even after, say, ten years pass, the ban will still be unenforceable in practice unless Congress also either:
1) Enacts national firearm registration, requiting FFLs to report all sales so they can be placed in a national database, and requires such reporting backdated to the effective date of the private sales ban, or
2) Makes firearm possession illegal, period, providing for a defense if the gun owner can prove they bought the gun before before the effective date of the ban, or bought it from an FFL after the ban.
If Congress doesn't enact (1) or (2), then a private sales ban could not be enforced. A prosecution would be required to prove that the possessor didn't acquire the gun through an FFL, and that would require checking the 4473s of every FFL in the State, quite possibly for years in the past.