There are loads of them listed here: Classified Ad Scripts. I guess it depends on your budget and what sort of hosting you have available. There are quite a few free scripts as well. Most of them require a bit of setup, but usually nothing too involved.
I think the number of webmasteres start making a classified site is decreasing a lot. But if someone really looks for a powerful script, paid scripts have better features and they can be used for professional websites.
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