This is an interesting question. I definitely don't believe there's an "easy" way to determine the percentage of true auctions, however. A person would either a) have to take the time to monitor every live vintage card auction on ebay for months, then compile the data and see if it is consistent enough to arrive at a percentage or b) somehow be persuasive or industrious enough to find out from an ebay representative (I place the odds of this at near zero)...
I will say it would be nice if sellers did have the courage to auction their
items on the site since it is, well, an auction site... I have used ebay as a last
option for almost 2 decades because of its many flaws and frustrations. One
of the challenges I voice to a seller who has annoyed me (rudeness, absurd
values or expectations, etc) is to have the courage to actually auction his
card over a 7-day period. The typical reaction is either crickets, or irrational
tripe that reminds me of why ebay is my last option for cards
Trent King