Tell you what the day you start listening to and doing whatever every anonymous caller tells you to do is the day you can say that anyone should do the same.
You are holding this store owner to a higher standard than is required by the law and you are also holding him to a standard you do not practice
Of course I would practice such a standard. If I had a gun shop and someone called to give me specific information on a dangerous person I would take it exceedingly seriously.
This wasn't an anonymous caller. It was the girl's mother. If the whole thing was bogus, no one died.
People take anonymous calls seriously all the time. An anonymous call can empty a mall, ground a flight or cancel classes for an entire school.
It goes without saying that the store owner was wrong and it cost them.