NOS, how often is this term used correctly?

I would guess this term is misapplied.. lots. I have bought / for good money / tubes that were barely servicable.(referred to as 'nos') One look at the marks on the pins makes me suspicious.I'm refering to power tubes. These have a shorter life / and some amps are harder on power tubes than other amps.I have the CJ 5's,and they allow bias for each tube individually.(all 16)This method is a pretty good "tube tester" unto itself.Paying big bucks to an individual is risky.I've seen used tube testers here for sale for around 500 bucks... Being in possession of such a tester does not an expert make. From everything I read Ram Labs & Upscale Audio do many tests most others don't do. Know your seller,be careful, and "good luck".

You probably see the term "N.O.S." abused the most at eBay. There are those that really know what New Old Stock means, they are the ones' listing mutual transconductance results, etc.

E-mail when in doubt. It doesn't take long to know who is knowledgable.

As with all high end audio purchases, consider the source as others have suggested here. Reputable dealers such as Vintage and Upscale will inform the potential buyer exactly the condition and age of product they sell as NOS.