I have to say it's my opinion now that, although it does matter a little bit about post count, you should never entirely trust someone because they have a lot of post. Personally I don't trust anyone any more, due to my number of bad trades (although not that many, considering I have 0 good trades currently it's not great), as my 5th trade, and most recent, was with 'this x' who at the time had 700+ posts on sythe, as well as 20+ legit vouches. He proceeded to scam me for $50, because I was foolish enough to trust him.
Long story short, it's important to have a reasonable post count, but should never be something that makes you trust someone.
Edit: If you go to his profile, by the link in my sig, you can see that he signed up in January 2010, which means he had achieved 700+ posts in less than 4 months. That just makes it completely clear how easy it is for someone to gather a lot of posts in a reasonably short amount of time.