Tony I wouldn’t worry so much about it if the seller will replace a noisy or bad tube. Your going to get bad tubes it’s just going to happen. I’ve been buying most of my tubes used from the same brick and mortar tube store for the last 15 years with great luck.
Much cheaper, I just bought some used12ax7’s that test new last week. RCA 60’s black plates $45, 60’s Telefunken smooth plates $55, 60’s Mullard Blackburn plant $45. el34 Mullard xf2’s $100 each. 60 day return no questions asked. Thats pretty much half off NOS tubes.
I've also use Brent for NOS he has always backed up his tube I ended up with a noisy 6922 bugle boy, he had one sent out same day, I do stay away from eBay. Find some good dealers and stick with them they will do you right.