primer on buying vintage tubes?

Dear all-

I recently started to tiptoe into tube rolling--replacing the 6922 stock tubes in my headphone amp etc.

I am curious as to why vintage tubes are often preferred to the new ones? Why would older tubes sound better?

And even more pressing, I'm curious about best practices for buying vintage tubes. I recently came across a pair on ebay that were not labeled NOS but the seller did say the had been measured on a tube tester and had 100% emission. If this a safe bet? Is it better to buy NOS? Is it better yet to deal with a vendor like Upscale Audio?

Thanks for any insights! Margot