I hesitate for several reasons, mostly being it won't prevent fraud.
A scammer can simply join a few weeks ahead (in preparation), make a few posts then continue as a con artist posting ads.
Also there are legitimate people who don't hang out on forums and want to sell on jemsite and not auction. To keep them at the curb doesn't benefit members either.
Everyone needs to use due diligence when buying, regardless of when a member has joined, how many times they've posted, etc. No classified screening would change that fact... glen