5
I'm sure there is a better solution and I'm not sure this would work, however, what I'd probably try is creating a one page module (easy to do), activate it but keep it from the menu (maybe keep it active for admin users). Place a "recent ads" block on that page and then figure out how to have an rss option for that page. It doesn't sound too difficult but I haven't tried it.
"I'm going to see if this internet thing takes off" - someone told me this when I suggested he needed a website.