Tube sellers

New Old Stock, so new not used. I've had good results with Brent Jessee and TC Tubes, so you tend to get what you pay for.

The better independent sellers will provide the tube testing numbers and will perform their tests on quality tube testers.

Buyer beware!!! - My friend just got ripped-off from a guy in California who used pictures of his vast tube collection (mostly NOS) that actually were pictures of tubes from other persons websites. My friend got his 2,000 tubes, but they were all non-audio tubes and complete junk.

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