Old threads never die- they just get resurrected..........
1. The "Shoot thru the Holster" shown is not a Federal NFA item, in that it does not disguise the gun. There WERE holsters for a derringer that wrapped all the way around, disguising it, with a hole for the trigger finger, permitting it to be fired while still concealed- and THAT made it an NFA item. The holster in the Flea-Bay posting does not do that. Whether it complies with YOUR state laws, I have no clue.
2. Ref: Being robbed, protecting your property, etc- Robbery is the unlawful taking of property of another by force or by placing them in fear of death or serious harm. It is NOT simple theft.
Wave a knife in my face, demand my wallet, you were placing me in fear for my life. Before I shot your ass repeatedly. But I shot you for making me fear for my life- not my wallet with $4 and a movie ticket stub.
I am not an attorney, you can do your own legal research, but I know of no state that forbids the right of self defense when attacked, or under a CREDIBLE threat of attack. Known as right of self preservation.