This paper contributes to the growing literature on the economics of green buildings: by merging auction theory and hedonic regression analysis we investigate the relationship between market concentration and price premiums in the American market for eco-certified real estate assets. Auction theory is used to model price formation where eco-investors may differ in their valuation of assets. Controlling for a large number of features, the empirical results provide evidence of a significant and positive relationship between investors' eco-certified market share and prices of eco-certified space. Contributing to the recent debate over the nature of the green premium, we find that eco-investors are creating clientele effects and that they may be subject to a green winner's curse.