What's the deal with buying used tubes?

[Herbie the Rad Dorklift]

?

I mean, you don't know how long they've had. I'm replacing the power tubes in my Mark IV because they're sounding old now (Over a year since new), but people will want to buy them? =/

 

[Restless Rocks]

You're taking a big chance buying "used" tubes from some goof on ebay. Some of the older preamp tubes can have years left in them but when a power tube (6L6, EL34 etc.) is shot, it's shot. Unless there were a way to verify that a power tube has VERY low hours on it and it hasn't been used in a bad minton game I wouldn't take the chance.