Have you guys considered raising the amount of posts needed to make threads in the marketplace? I know it may lead to people trying to SPAM etc. But seem as you are enforcing/moderating the forum more effectively now. It may weed out even more useless sales threads. However could equally mean more useless threads elsewhere..
The post limit was a terrible system, people would make ten (the former limit) posts and be in almost immediately. We were constantly left with lots of crap posts per week which significantly reduced the overall quality of the forum. We decided on the four day limit since there is nothing that can get you around it. It has worked better, but not perfect.