---- Acknowledgments ----
We appreciate support from the National Science Foundation, the Stanford Institute for Economic Policy Research, the Toulouse Network on Information Technology, and the Alfred P. Sloan Foundation. The views expressed herein are those of the authors and do not necessarily reflect the views of the National Bureau of Economic Research.
---- Disclosure of Financial Relationships for Jonathan Levin ----
I received compensation as a consultant to eBay's research group in 2010-2011, and subsequently have done research under a data-sharing agreement with eBay.