Zillow plunges (Z -5.4%) after becoming Citron Research's latest target (.pdf). Zillow's valuation (16.5x trailing sales) and insider sales receive scrutiny, as do its heavy sales/marketing expenses (44% of Q2 revenue) and slowing unique visitor growth. The short-seller also highlights a Citi survey indicating slowing online real estate ad spending growth among agents, and subdued enthusiasm for Zillow's Premier Agent service. Trulia (TRLA -3.2%) is also off.