Any guidance for buying used tubes?

Used tubes can be valuable if you are just starting to roll tubes and are not sure exactly what you are looking for. Costing far less, you can experiment until you find the best match. Then buy NOS. Doesn't make sense buying expensive NOS if you are not sure you will like them or not. Then you're in the same boat as the guy you're looking to buy from.

There are some good tube dealers that carry used tubes, like- Charley at "vacuum tube valley",and I think Andy at "vintage tube services" does too.

Used tubes aren't worth much unless you are just buying cheap to test them out. Be careful of shorts before putting them into your amp/pre.
I would buy used from a good dealer if they have been computer matched. However, it's usually not worth it.

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 agree with Dopogue
N.O.S. stands for new old stock, these tubes are to start with 40 years or older. How can one be certain they are new. What is perhaps more important is how well they test as to how much life remains in the tube. Used tubes that test strong (depending on application) can last longer than we keep the equipment. I have many tubes that test strong sound great but I would never state that they are N.O.S. perhaps they should be called N.N.O.S. nearly new old stock.