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About.com--then called The Mining Co.--tried classifieds a few years ago, but the experiment was a failure (probably because the ads cost too much for the number of impressions and clickthroughs that they generated).
FWIW, I just tried clicking the links in the classified-ad boxes at several About.com sites, and the links didn't work. That doesn't exactly inspire confidence. :-)
I think the real challenge for About will be competition from its own Sprinks. Why pay for impressions when you can pay only for leads?