You can't dive or use any equipment without air/nitrox. I'd say every diver NOT KNOWN TO A SHOP/FILL STATION should be carded (initially) for a tank fill or to rent a filled tank. Without one of those two, you can't dive. I see no need to 'card' a diver for any gear purchases. Put the 'check' into the system where it's guaranteed to work ... at the one element required on every dive ... compressed gas.
Or ... don't worry about it at all. It's their life on the line. We get certified because we want to learn to dive safely and competently (at least that's why I got certified), not so that we can show our card off. I've rarely been 'carded', but it's typically by the dive operator (quarry or charter boat), only occasionally to get a fill.
Funny how so many people will blame others for their own faults and sue you for selling them air...!!! I would never take the chance if I was a shop owner...! lee