For a bilateral trade model with a privately informed buyer we characterize trading rules which are implementable via a mechanism with ex post renegotiation. Let R(v) be the seller’s expected payoff if she asks for a price of v, and let ~R be the least concave majorant of R. We show that any non-decreasing trading rule can be implemented subject to two constraints. First, on any interval on which R(v) < ~R(v) the rule must be constant. Second, it must be equal to 1 above p*, the highest maximizer of R.

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