The number of homes listed for sale on Zillow was up 17.7% year-over-year in June. Inventory was up for the fourth straight month and climbed in 81% in the metros covered by Zillow. But overall inventory remains tight. "Tight inventory is especially pronounced in the lower end of the market, which we define as the bottom third of the housing stock," according to Svenja Gudell, director of economic research at Zillow.