1. Department of Economics, University of Washington, Box 353330, Savery 305, Seattle, WA 98195, and NBER (e-mail: )
2. Department of Economics, University of Texas, 2225 Speedway, Stop C3100, Austin, TX 78712 (e-mail: )
3. Haas School of Business, University of California Berkeley, 2220 Piedmont Ave., Berkeley, CA 94720, and eBay Research Labs (e-mail: )