How many used cars are sold in a year? It's got to be more than a million, doesn't it? So, yes, that $15 million Porsche drives up the average price... by $15. That's not significant.
In the US, ~45% of car registrations are new vehicles. In 2016 around 17.6 million new passenger vehicles were sold in the US with a combined transaction price between $500B and $600B.
That puts used car sales (including private sales) at around 22 million units, and nearly 40M total cars sold and registered in the US in 2016.
Regardless, if someone wants to complain about expensive classic cars, Porsches aren't the ones to complain about, with most (bulk being 911s from '78 - '93) trading hands well under $100K (964s being more like $50-65K, SCs more like $20-30K) despite the Porsche bubble that's been growing for a long while now -- since Ferraris long ago got too expensive for first time classic car buyers.
$15 due to a single sale seems very significant. Correct me if I'm wrong, but by your reckoning just 100 super collectable sales (average $5m) could raise the overall average by over $500.
