4
Are you wanting to charge people for listing items, if not why not consider MyAds. this could prob be tweaked to your needs
and I'm sure that wouldn't be a hard task.
If your determined to have the script you linked to, then you could also ask them, I see they offer custom work, However i see a price tag on that software so licencing would prob be different to XOOPS GPL, and would obvious mean it could not be distributed, so you maybe need to consult with them first anywayz. just a thought
Hope this is of help.