Any guidance for buying used tubes?

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.

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.