New in the box or NIB should mean exactly that new and unfired , it is still second hand , which can cause warranty problems, and if in current manufacture should be sold for approximately wholesale price , unless it has a rarity value .
Used or like new is in the eye of the beholder , both seller and buyer , invariably the seller will have a better view of the firearm than the buyer .
Unless the firearm is rare , not uncommon , the value depends on what the buyer is prepared to pay and the seller prepared to accept .
I had a Uberti Hawken , not rare but uncommon , as it was a modern made replica it is not collectable unless coveted by a Hawken hoarder
. I sold mine for slightly more than I paid for it 30 odd years ago , it was in good used condition ,metal work and bore used but excellent , stock with a few dings and marks . With inflation etc I was happy with the price I got and so was the buyer .
If you are going to buy a second hand firearm , check the wood for dings, fit , finish and repairs , look down the bore and along the outside of the barrel to see if there any bulges or dents , and that the rifling is fine . Most importantly check the screw heads , check for burrs caused by screwdrivers which don't fit , make sure the screws are all the same age and design .Screws are easily replaced but damaged ones do represent some butcher has been at the gun , . look for marks on the metal surfaces which just should not be there . In the case of a shotgun take the barrel out of the stock and tap the barrel with a screwdriver handle or some such , the barrel should ring like a tubular bell . If the seller wont let you do these things walk away , unless you don't mind the dings and marks and just have to have the gun , then that is up to you , use small fixable faults, like burred screws, as a bargaining tool