This is definitely a term of art ... not as one writer noted it as "absolutely worthless" as a description.
It describes inventory that is no longer in production or available for retail sale, but has never been sold to a consumer.
A consumer could buy used, but if New Old Stock is available it is preferable to used.
Ity is possible that seller misuse the term, but when I'm looking to buy discontinued items, I love to find NOS!
Buy It Now