Tube sellers

Agree with Thorman. NOS should be taken with a 100kg grain of salt. The important point is the actual performance of the tube, not if it has or has not ever been used. In fact, I suppose its possible for a 50 year old tube to go bad even if it has never been used.
Long term (short term I hope) we need to figure out how to manufacture tubes up to the quality of the old classics. The old tubes aren't going to last for ever, and some of them are going for really steep prices already.

Often it means NOISY old tubes that nobody wants.

To me,NOS is just what it says NEW old stock. I agree with you that a lot of tubes being sold as NOS are probably used and test to new parameters but they are not NOS. You have to find a tubemeister you can trust. How do you do that? I have no idea,I guess faith will have a lot to do with it !!!! I have a lot of faith in mine.

NOS means just what it says: New Old Stock.
How much you believe the tube is actually NOS depends on the tube.
NOS Mullard EL34? Maybe not.
NOS CBS Hytron 807? Good chance it is. And many rectifiers.
Just my opinion of course

There are NOS tubes out there, but the supply is dwindling fast. It is important to buy from a reliable seller. I have had good service from Kevin Deal at Upscale Audio. NOS tubes come and go and when there are fewer tubes left the price goes up until they are gone. If you snooze you lose.