As long as it has not been fired by anyone but the factory it will be considered NIB. You will see " used" guns that have never been fired advertised as NIB.
It will be difficult to get any discount without visible damage.especially on a somewhat rare gun.
As far as the slide release.remove the magazine and it will be easier. It will be easier with a loaded mag or no mag. The pressure of the mag spring and follower make the release difficult to operate.