Once in a blue moon, you'll find a gun that is mis-priced far below market. At such times you have to jump Now. The window of opportunity usually is open for only a short while.
Of course, there's no telling if you'd have said you'd take the gun at $600 when you first saw it, that they would have honored that price. But, usually if it is actually marked that way out in the open, stores honor posted prices.
I've been in similiar situations, sometimes I've jumped on it. Other times I didnt - went back later only to find the gun gone, - sold in only a few hours.