The gun classifieds require 25 posts on the forum to promote your user status. The server will not always do this instantly, but it does do it automatically; so you may have to wait a few minutes for it to do so.
If you cannot make 25 posts on the forum, and/or are too impatient to do so ... please do not email me asking why you cannot post your stuff for sale on the forum. The forum survives off of people participating in our discussions, and the classifieds are a service for those that participate. Furthermore, the majority of buyers on the forum want to know a bit more about the person they are buying from, so it's in your best interest to join in on the discussions.