It's everyone's responsibility to clear a gun before handing it to anyone, it's then the responsibility of the person being handed a gun to make sure it's cleared again. I make a point in a gun store to clear the gun in front of the person, and expect to see it checked again when I hand it over.
Guy apparently wasn't adhering to the first rule, never let the firearm point at a person. That rule trumps all accidental firings.
But someone working in a gun store should consider it gospel.
That's what business liability insurance is for.