The $3.5 billion purchase of Trulia (NYSE: TRLA) by Zillow (NASDAQ: Z) will create by far the largest online real estate information company and presents a direct threat to the National Association of Realtors, which is closely affiliated with the website Realtor.com, the distant third-place site in terms of web traffic. Many real estate agents worry that the consolidation of the two online market leaders will translate into higher advertising costs.