This is anecdote, but it might be useful to some.
I hang out with a bunch of younger gearheads most Tuesday nights over pizza and beer. A favorite pastime is scrolling through bring-a-trailer and other car dealer sites on phones, with interesting vehicles for sale passed around as memes. Each of these guys owns upwards of two cars, some own 5 or more. Never once have I heard of them buying a vehicle from any on-line site, except in one case from an obscure estate auction outfit. All their used cars were bought private party via craigslist or word of mouth.