This gets us one step closer to getting the answer to the question:

State Law Trumps Fed?
or
Fed Law Trumps State?


Is it the United _States_ of America?

or

has it become something else?

Selling a firearm retail is a federal jurisdiction
possessing a fireman appears to be a state jurisdiction

My take is that if you show up at your local gunshop wearing a blaring green "420" t-shirt, you may not be able to buy anything.

As mentioned, I doubt the feds could get a list of cardholders.

I do not doubt that if the fed wanted to know if somebody was a cardholder in conjunction with a crime they would have no problem getting confirmation as long as they didn't pry to find out exactly what condition the cardholder qualified for.