I personally believe that post count does mean something in terms of trust, but not too much. If a person had below 50 posts I would most likely not trade with them. If a person had above 50 posts, I would most likely trade with them, depending on what those posts were about (making sure it's not just spam, like "free bump!" or "nice acc", join date, and vouches, of course.
I look at vouches mainly, and who they are from; I will admit I do also look at post count and join date, scammers typically have low post count and a very recently join date. The majority of scammers join, post around a little, scam, get banned and repeat on a different account.
Then again, it is easy to gain post count, as seen with some members who can gain over 200 post count in just a day.
It doesn't mean that much.. Though people will trust a member with 900 posts earlier then someone with 50 post.Probably because the 900-post member did spend more time on the forum.
You really have to consider everything, cause a person who has an account from 2006 with 190 post count could have made it, come back so they can say they are old member, high post count short time could mean spammer, so they have high post count therefore they want you to go first, but you really got to consider everything to tell if some one is going to scam, so you can't put a value on post count.
I look at post count, Vouchs, and when they joined. Also I search for threads and I look up their email for other game websites in case they scammed someone. If they did I dont trade them