Any guidance for buying used tubes?

Preamp tubes, to be a bit more specific. If I get info from the seller regarding amount of use, what else would be helpful in deciding whether to purchase?
Buy them from a reputable dealer. Try Brent Jesse Recording. He has an ample supply of tubes. He sells new, NOS and used tubes. They are rigorously tested and he guarantees them. He has a very user friendly website.

I will not buy used tubes from a seller that won't stand behind the products they sell. I have been burned by well intentioned sellers on eBay and Audiogon. They sing all kinds of praise about their tubes. Two weeks later one tube fails. you give them a call and they try to blame you or your equipment. BTW, I don't work for and am not related to Brent Jesse Recording. Good Luck.
Rule #1- Know your seller well.

Rule #2- See Rule #1.
Don’t! Buying used tubes is like buying a used Tooth Brush. Stick to NOS and New from dealers you trust. Sometimes you pay a little more but a good dealer provides service, such as advice and testing.
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 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.
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.
If I were buying used tubes I would ask the seller if he has tested them and what tester he uses. Some people that sell tubes are liquidating their stock because they changed gear. With the exorbitant prices of tubes I would be more inclined to buy used if the seller answered all my questions to my satisfaction. Some sellers that espouse to be "tube gurus" have sold tubes that are faulty and the sales pitch is over the top. Bottom line is, will the seller stand behind the product. With the decreasing avaliabilty of NOS tubes "used" will play a more important role.
A raft of great advice. With regards to testing, I've seen some test results that are described in just a number like 70 or 80, that is related to some kind of percentile value, and then other test results that are "x - y" where x is mA and y is mA/V. How do these methods relate to each other? Are they saying the same thing?
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 :-)

Call this number 616-454-3467 his name is Andy. He also has a website but forgot the address. The only guy to buy tubes from. Prices not bad neither. Highly recommended!
Brent Jesse Recording
Tejas Tubes