Here in Florida, I took my California-resident son-in-law to a gun store/gun range not too far from where I live. One of the salesmen offered to show him some weapons. When I mentioned that Brian was from California, the guy actually jumped back and said, "Man, you're kryptonite in a gun store."
So, it would seem that at least some gunstore personnel are aware of the strictures regarding sales to out-of-state residents.
And California has long had prohibitions on AKs and ARs. Even in the C&R pubs frequently they have a "Not Available In California" tag on any weapon that holds more than 8 or 10 shots in the magazine (I apologize for not remembering the exact number) irrespective of their historical status.
Sounds to me like Cavalry Arms got a little greedy, but I will reserve judgement until all the facts are in. Maybe Shawn thought the dude was from Mexico???