Cheap Talk in Bilateral Trade

2026-05-31

AI Summary

This study addresses bilateral trade with a seller possessing private costs and limited to price commitments, where traditional mechanisms struggle to simultaneously achieve incentive compatibility, individual rationality, and enforceability. The paper introduces multi-round cheap talk into a single-buyer bilateral trade model for the first time, integrating Bayesian game theory and mechanism design to characterize optimal pricing under limited commitment. The analysis reveals that cheap talk is ineffective in the single-good setting; however, in extended environments—such as multi-good or multi-unit trades, settings with interdependent valuations, or repeated interactions—cheap talk substantially enhances expected profits for both parties and consumer surplus. Moreover, multiple rounds of communication strictly outperform a single round, highlighting the distinctive value of iterative cheap talk in complex trading environments.

Abstract

A single seller offers one or more goods to a single buyer. The buyer's values and the seller's costs are private information. Each player has a commonly known prior over the other player's value or cost, supported on a finite set. What is the optimal selling mechanism? We argue that, despite this question's importance and apparent simplicity, prior work offers no satisfactory answer. If the seller simply chooses an optimal menu given her realized costs, she fails to exploit her informational advantage. At the other extreme, the optimal trade mechanism that satisfies IC/IR constraints for both parties fails in practice, as it conditions prices on the seller's unknown costs in an unenforceable way. The seller's realistic capabilities lie somewhere in between: she may leverage private information but lacks unlimited commitment power. To bridge this gap, we consider a solution concept built on the realistic assumption that the seller can commit to prices but nothing more. Similar -- albeit technically distinct -- solution concepts have been studied in the context of auctions with multiple buyers. Our concept proves surprisingly rich even with a single buyer. In our model, the buyer and seller engage in multiple rounds of cheap talk before the seller posts a menu of priced bundles. The buyer then purchases. We measure value as profit for the seller and consumer surplus for the buyer. We prove that with a single good cheap talk cannot help either party, but show that it creates value in any extension of this canonical setting: multiple goods, multiple units, interdependent values, or repeated play. We also show that multiple rounds of communication can yield strictly higher expected profit than a single round. Finally, we discuss how realistic factors beyond our stripped-down model combine with cheap talk to enhance this value even further.

Problem

• Cheap Talk
• Bilateral Trade
• Private Information
• Mechanism Design
• Commitment

Innovation

• cheap talk
• bilateral trade
• mechanism design
• limited commitment
• multi-round communication