Any guidance for buying used tubes?

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.

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.