Tube sellers

Personally,I don't think its very likely that you will recieve many absolute NOS tube..How can one know ( unless they bought them new years ago ) ..if they have or have not been used..I do feel ( and have bought many NOS tubes )there are many relaible tubes sellers ( and dealers ) that test tubes and they test in the " NOS " area of testing parameters..Some tubes dealers have NOS tubes that are still in the Original boxes and test in the NOS area,so I suppose its possible that some are NOS..To me the most important info is how the tubes test and how quite they are..If they test in the NOS area and are quiet and to me thats ok...

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

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.

As Thorman implies, most vintage tubes are more properly referred to as ANOS (almost new old stock.) No, I'm not joking. And I prefer ANOS when buying vintage tubes because they at least have some track record. If they were going to fail, they already did. If they were going to develop microphonics, they already have. Their test results are stable and won't suddenly change after a few hours, as is often the case with brand new tubes; either vintage or contemporary -- and which is why there's really no point in testing brand new tubes right out of the box. Most reputable tube sellers burn in new tubes for at least 24 hours before attempting to match them.
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My expectations are that an "NOS" tube will test the same as a new tube would and have a long life. I have noticed that some dealers are now selling tubes with very little flashing/gettering which indicates to me heavy usage. Others are just stating that tubes test "as new", which as Thorman said, is really the most honest sounding representation anyone can make.
I have often wondered how a dealer can have very similar appearing tubes and call one batch, "used test new", and the others as NOS. It is unlikely these days that anyone can source these precious old tubes in sealed cases that came from the manufacturer. The only time you might see that, for the most part, is with tubes that aren't used for audio.
BTW quietude while a reasonable expectation is not guarunteed by being NOS.

'NOS' tubes are one of the bigger problems we have with our preamps. It is very difficult to find NOS 12AT7s that are as quiet as actual new ones. As with any high-performance tube equipment, you can't just buy tubes off-the-shelf and have them work 100%. Usually you have to hand-pick them.

So in practice, when people have bought NOS 12AT7s for use in our preamps' phono sections, they are often buying tubes that are significantly noisier than what we regard as acceptable- then they assume that the preamp is noisy! Before we even audition a tube, we start by grading it in a tester. It must read about 80% higher than the 'passing' value to even be considered for use. Incidentally, that number is typical of 'testing like new' for 12AT7s, anyway...