The clue is in the title...
...as in, unused and in the original packaging; I would allow a dealer some leeway if they had something sitting in a box on a shelf that came in bulk ( like BB's, brake pads, tubes, etc...) that they can sell individually as 'NOS' because it, quite literally, is...
...if it's something a private seller bought but never used it isn't 'NOS' it's 'Second Hand.' 'As new' maybe, even 'Mint' but definitely not 'NOS' because, quite simply, it isn't
If a private seller buys up a dealers' old stock then they can legitimately advertise it as 'NOS' because, in this instance, it is
I will be selling some decent kit I bought as 'NOS' that was very 'NOS' and boy did I pay for the privilege!
I fitted some of it (it's on my Explosif in the link below...) then had a major rethink on the direction the build was going to take and removed it again. The parts have been ridden exactly 10 miles and do look unused, but I shall be selling them as 'Mint, as new condition...' because that is what they are; I shall also be taking a hit in the wallet due to my own impetuous ebay habit!