stef25 - 2:41 pm on Jun 3, 2010 (gmt 0)
Joe Sixpack can post the property he is selling or renting out for free, agencies pay money to put their listing up. There is also a lot of (obtrusive) advertising on the site. I'd imagine those agencies pay to put their listing up on all major real estate sites.
They advertise their site via Adwords and other channels but AFAIK nobody is paid to put up content, on the contrary.