1. As far as posting an ad you would need to go into the classifieds admin panel and under EDIT USERGROUPS you would set permissions as you see fit.
2. I noticed looking at your classifieds your forum header is not showing. Thats a simple fix as well edit your global style settings and the titleimage variable you have a relative url to the logo like so.
You should make that a full url if you want it to show over another directory like classifieds http://www.gusty.se/forum/images/custom/forum_logo.png
You also want to edit your header template in vb
and fix any relative urls to other images like this one.
You need to correct those to full urls also.