The platform design, the process that helps potential buyers on the internet navigate toward products they may purchase, plays a critical role in reducing search frictions and determining market outcomes. We study a key trade-off associated with two important roles of efficient platform design: guiding consumers to their most desired product while also strengthening seller incentives to lower prices. We use simple theory to illustrate this, and then combine detailed browsing data from eBay and an equilibrium model of consumer search and price competition to quantitatively assess this trade-off in the particular context of a change in eBay's marketplace design. (JEL D12, D44, D83, L81, L86)

Auctions were very popular in the early days of internet commerce, but today online sellers mostly use posted prices. We model the choice between auctions and posted prices as a trade-off between competitive price discovery and convenience. Evidence from eBay fits the theory. We then show that the decline in auctions was not driven by compositional shifts in seller experience or items sold, but by changing seller incentives. We estimate the demand facing sellers and document falling sale probabilities and falling relative demand for auctions. Both favor posted prices; our estimates suggest the latter is more important for the auction decline. Survey evidence provides further support.