They could enforce it exactly the same way they enforce liquor carding laws. Periodic stings with undercover officers trying to buy to see if sellers comply. It doesn't prevent 100% of crime (nothing does), but it's not true that you need a registry to have some sort of enforcement mechanism. There are other ways.
Godwin's Law? I'm talking about getting some things we really want (CHL state-to-state reciprocity, or a change of may-issue statutes to shall-issue) in exchange for non-registry UBCs, which are minimally effective
but don't hurt us. I'm not talking about genocide here. Nazi analogies aren't needed.