Why is my post count changing?
First, there are certain areas of the forum where posts do not count toward your total post count (i.e. Games, Introductions, Newbieland). No matter how much you post there, your post count will not increase.
Next, there are areas that are routinely pruned. For example, the Marketplace is frequently purged of sold items, threads are removed from the galleries occasionally when the image links no longer work, and threads are deleted from Doll Arrivals! after three months of inactivity.
If you already have marketplace access, and are an active member, dipping below the required number of posts will not affect your access level. The only way that you can lose access is if an administrator changes it manually.