There is a buzz abuzzing around Edgeio, which, according to this entry [Via /Message], will use tags and blog crawling to gather classified ad information right out of your blog and manage it online to help connect you with buyers.
Ad listing in blogs? Who does that? Today, almost nobody. And that's why this idea could work at all: Teare said the tag "listing" is found only about 10 times a day on millions of blogs, so it's an ideal, clean tag with which to create a unique index of "listings from the edge."
There appear to be two risks in this model (I haven't yet ready any critical descriptions of the system - everyone blogging about seems to have been positive and/or had some relationship with the company). Firstly, no-one owns tag space. This is one of the biggest failings of tags (and one of the most predictable). By not having a formal system for meta-data, there is no control over meaning. So now the tag 'listing' means 'I want Edgeio to crawl me, interpret somehow what I have on my blog and then offer that to buyers'. The argument against formal spaces in tags (a la XML name spaces for example) is that they require either more work on the part of the user or integration into blogging platform as a standard (oops - didn't Technorati achieve that with their ownership of the tagging mechanism - perhaps that would be too meta).
Secondly, the value of such a system to sellers requires pretty good coverage. I'm guessing that Edgeio only crawls RSS enabled blogs. This means that there will be some percentage of bloggers out there who cannot list on Edgeio. In addition, given the unstable state of the ping/RSS duopoly, Edgeio will have all the challenges of any other blog search engine in keeping things up and running. This risk may result in value - more reason to improve the ping/RSS stability and coverage, but it may not.
It's going to be interesting to see how this stuff pans out. Good luck to them! Somewhat Frank has a more detail explanation of the service.
BTW - imagine what would happen if someone wanted to put out a DOS attack on Edgeio by flooding the blogosphere with the listing tag. Will we enter a new era of spam functionality?