Tube sellers

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.

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.