As someone who likes to lurk almost exclusively in the marketplace, I can attest that the policy works rather well. While not a major trend (from the public view), there were a number of scams taking place (trades where one end didn't send their item, no communication after payment, sending broken item, etc) that involved larger sums (>$1K) of money. The occasional scam still happens but it is more of an anomaly than a trend.
The post count and time requirements seem to deter most scammers, and I think it serves as a good trial period for posters to acclimate to the forum posting guidelines. If anything, the biggest current issue with posters in the marketplace is that some are a a bit overprotective at times in making sure other posters don't overpay. I'd much rather have that issue than worrying about scammers.