They have turned Madison, a city of 208,000 known for its liberal politics, into one of the most active for-sale-by-owner markets in the country. And their success suggests that, in challenging the Realtor association's dominance of home sales, they may have hit on a winning formula that has eluded many other upstarts. Their site, FsboMadison.com (pronounced FIZZ-boh) holds a nearly 20 percent share of the Dane County market for residential real estate listings.