Yet I, as a child, "somehow" survived the nonexistence of car seats when we were kids in the 60s, and utterly intact. My parent's were very cautious drivers.
That doesn't change the fact that some drivers out there aren't cautious at all.
I wouldn't buy a secondhand anything from anyone I didn't know and trust where a childs safety is an issue.
That's not a very rational position to take.
Was your house built entirely from new, and only by people you knew and trusted? Is the same true for everything in your house that might possibly harm your kids? Do you only feed them with food grown by people you know and trust? Getting closer to car seats, was your car built (and is it maintained) only by people you know and trust?
It is possible the answers to all those questions are "yes", but most kids get along just fine without such paranoia. In fact they may be better off learning how to evaluate risks for themselves (and making a few non-fatal mistakes along the way) rather than living in an artificial bubble.