I have tried PHP Classifieds and like it already. The only thing is that the user integration into PNuke would have to be programmed... Unfortunately I am not capable of doing this on my own...
You can see the PHP Classifieds script website here:
The script is very very stable and easy to install vie install.php. Sets up everything on its own. Easy to configure and to adapt to Nuke, I believe. If someone would like to take care of it... I am sure the community will be greatful.