Any guidance for buying used tubes?

Trouble is, it's often REAL hard to tell used tubes from new ones, visually, and
even harder to believe that the bulk of the ones SOLD as new actually are*.
On the other hand, I've bought a lot of used Old Stock Mullards, Telefunkens,
and Amperexes that test just fine and last a l-o-n-g time. I don't mean to
imply that there aren't wolves ready to pounce on poor tube buyers on Ebay
and elsewhere, but your chances of getting good used tubes there aren't
hopeless either, unless you have very bad luck. After at least 10 Ebay and
Audiogon tube buys, the good ones have far outweighed the bad.

* I'm not trying to bad-mouth the reputable dealers here, although I do
wonder sometimes how they can be 100% sure they're selling NOS tubes
(when they advertise them as such).

I know what you mean. The seller seem to assume that you have (or at least know the specifics of) his particular tester and its numerical values, not to mention its quirks and nuances. Also, some tube testers, such as my Eico 667, have multple settings for the same tube type, depending on whether you use the integral roll chart or the various printed charts and whether you test for 12AX7 or ECC83 (supposedly identical). I have a "choice" when I test my 12AX7s, for instance, and you can bet I'd be tempted to pick the one that shows the tubes in the best light. Not that I'd EVER give in to such temptation, you understand :-)

Dave