Now that I re-read my original post I guess it does seem to lump Zillow and Trulia together too much. Again, my apologies.
Zillow, like Trulia, also invests a lot in developing their product to prevent, detect, and remove scam listings. To not do so would make the product unviable.
Popular sites are locked into a constant arms race with people seeking to defraud other people. Everyone's gotta make a buck somehow, and fraud is just the unfortunate dark underbelly of commerce.
Edit: Full disclosure—I do work at Zillow, and many of our hackweek projects are also focused on scam detection and prevention. Our objective is to provide the most robust and reliable service to consumers and to connect them with skilled agents, rated by other consumers. In two years I've never seen anyone scoff at a customer complaint or suggest we should ignore something until faced with the threat of "bad PR". I love working at Zillow because, above all, it's full of very decent people. And personally, I'm excited for a potential Zillow/Trulia merger because we all admire them a lot, and we certainly have a lot to learn from each other.
Maybe it was a fluke. But I don't trust Zillow.
This is really cynical and, honestly, just not true. Last year, for example, we (at Trulia) rolled-out enhanced rental fraud detection that, overnight, took down the number of published rental listings by a significant amount. And many of our metrics -- leads sent, value created for agents/landlords -- are functions of our listing count. We didn't remove these listings -- which nobody had flagged as spam -- because of some murky concept of bad PR. We removed them because we're building a RE platform, we want and need and strive for happy, repeat visitors. And because in the chicken-and-egg of a 2-sided marketplace, the answer at Trulia is clear: the consumer came first. We need happy, engaged consumers, not mindless page views -- banner ads are a small and shrinking share of our total revenue.
It's true that we as an industry -- and now potentially as a combined company -- need to continue to build more sophisticated systems to combat fraud. It's a topic here that gets a lot of discussion and mindshare, and there has been more than one fraud-related innovation to come out of our quarterly hack-weeks.