people act as if it's racism or something "oh noes you discriminate agenst post count!!!"
look. join date, post count, vouches, all are important in determining if this random stranger you're about to trade with is legit. he's from 2007? he's managed to go 3 years without being banned, must be good. 1200 post count? he either is a spammer, or cares about the community, would he trade that for a couple mill? vouches, the only ones that should count are serious trades, not bullshit like training some guy up to 15 mining for free. sadly all one would have to do is bot 5 people up to 15 mining for free, the people would say "vouch for ____ trained very quickly" and there you go, 5 seemingly useful vouches.
no matter what, people can either be legit, or scam. there's nothing at all you can do to 100% ensure your safety, because even OMMs can scam, at any point. it's just a matter of what makes you most comfortable.
tl;dr:
post count matters, but not nearly as much as join date or vouches, and both of those sometimes won't matter.