Vouches are clearly important when it comes to trust. The fact that you've already gone through trades without any issue of you scamming is great. However, vouches for training someone to 15 mining are useless, and shouldn't even be considered vouches in my opinion.
Join date is slightly important, depending on your activities here. If you're 2+ years old, and are clearly active in the Black Market section, there's a good chance you're pretty trustworthy, because you've lasted 2+ years doing trades and not scamming. However, if you've been here 4 years but spend your whole time in the botting section, there's not that same guarantee.
Post count isn't very important, at all. It COULD be considered as your role in the forums, you've maybe made a lot of posts buying accounts, and re-selling, or buying gold and reselling, or anything of that sort, and be super-trustworthy. However, you could also post random things like "inb4lock" or "1st" and still get post count up, but still scam people.
It's all situational. Vouches are the best, because if you've spent years here doing trades, or gotten 500+ post count on black market forums buying/selling stuff, you definitely have some vouches.