I think Amon_RA hit the nail on the head, to quote the definition.
Just to pitch in, I've sold many items (chips, parts) on this very forum advertised as NOS. The reason being is that my stock came from a defunct computer store that used to sell Amiga stuff in the 90's. And my stash was the remains of that stock sitting on storage shelves, unsold, for years. They were in various conditions, some stuff worked some didn't, some was discolored, some was pristine.
The ONLY point being that if the actual stock hasn't been sold, then it didn't get into private hands, hence there is less risk of an incomplete item, hidden faults due to usage, or coffee/moisture/pet damage inside, etc etc..
So NOS has really not much to do with if the item was opened, shrinkwrapped, tested, untested, broken, new, yellow or white. It's just that, what is above in the definition and people can chose to believe the seller or not.
In my case apart of NOS, I've always listed what condition the item was, if it was complete and if it was tested and working.
Really there is no point trying to affix any significance to this. Just read the description, look at the pictures, check out the seller and decide - and that's it.