There are actually two different things being discussed when one says NIB. One if the condition of the gun, and the other is the condition of the entire package.
Now, NIB should be used only when the gun is never previously sold, never fired outside factory testing, has all accessories, paperwork, etc, and shows no handling marks.
Anything else should be described as ANIB (as new in the box) or LNIB (like new in the box). One can find a gun "NIB" where the box shows wear, or damage but the gun is pristine. To a collector this makes a significant difference, but to a shooter, who only cares about the condition of the gun itself, it does not. It is entirely fair to describe a perfect gun as NIB (if the gun is perfect, never before sold, and unfired), with a damaged box, or missing accessories, PROVIDED it is stated what is damaged or missing.
It is up to the ethics of the seller to explain if they are describing the condition of just the gun, or the whole package. They should list such an item as ANIB or LNIB, but those descriptions also include guns that have been sold and fired.
around 20 years ago I bought a NIB S&W Model 28. It was made in the mid 70s. But it was still NIB condition, with box and everything. I considered it fairly represented, but many would feel it should not have been called NIB.