As I was just looking through the classifieds I noticed several other posts by members with 30+ posts, have you tried posting since you've surpassed 30 posts?
As long as you are trying to post a new thread in the classified section it should work for you, as I am having no trouble with it. However, if you are trying to post in a thread that already exists in the classifieds, that you won't be able to do as all transactions must be done through PMs.
There is no post amount you have to reach before posting classifieds here. I'm not exactly sure why you are not able to... Exactly what message are you seeing when you attempt to post? Thanks