But did you climb into it on the seller's property and fly it away? They aren't cops, but they should still act like reasonable people. In most areas there is no law about selling scuba equipment. Scuba licenses aren't handed out by governments. But good luck buying regulators or getting a tank filled from a proper shop without showing some form of proof you aren't going to go kill yourself.

I've purchased scuba equipment and tank fills from dive shops all over the world and have seldom been asked for a certification card.

So have I, but we probably give the impression we know what we are talking about. Send a 16yo kid who doesn't know a thing and see if they sell to him. That happens at motorcycle shops every day.