Classifieds via blogs are the latest buzz. Others wrote already about edgio et al. ( namics blog, Bernhard Seefeld and Andreas Göldi (from december and in german, still interesting)), so I don't have to repeat it here.
While those services are not yet public and don't have to show much right now, I assume they won't be much more than focused aggregators (with some goodies of course). Edgeio for example said, that they look for the tag “listing” and aggregate those posts. So, if this takes off and kind of becomes a standard, it would be easy to piggyback on that for others, for example Planet Switzerland (search for tag/listing). Throw in the hListing microformat, some georeferencing and maybe some more commonly used tags and done is the classifieds aggregator for blogs.
As others pointed out discussing edgio, there's a big spam problem in this idea (for which I don't have a good solution either) and the usual chicken and egg problem. A big part of the blogger crowd doesn't care about tagging nor about microformats, so either you have to have a pretty smart aggregator, which recognizes blog-classifieds without those tags or you won't get much classifieds at all. Furthermore I don't know how many people would try to sell their stuff via blogs instead of going to ricardo or ebay with a much larger audience. On the other hand, the blog community, especially the swiss one, is something like a fenced garden, one knows each other and this makes it a lot easier and trusted to sell/buy/find something.
Maybe I'll extend Planet Switzerland some day with some more features into that direction (hListing parser for example and some georeferencing magic). Anyone interested at all?