We show that a necessary and sufficient condition for entry to be unprofitable in markets with adverse selection is that that no buyer type be willing to trade at a price above the expected unit cost of serving those types who are weakly more eager to trade than her. We provide two applications of this result. First, we characterize cases in which market breakdown occurs, thereby generalizing the main result of Hendren (2013). Second, we characterize entry-proof tariffs on nonexclusive active markets, thereby generalizing the main result of Glosten (1994). Our analysis paves the way to new tests of adverse selection, notably besides the case of inactive markets studied by Hendren (2013).
 

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