"We provide experimental evidence that contractual incompleteness, i.e., the absence of third party enforcement of workers’ effort or the quality of the good traded, causes a fundamental change in the nature of market interactions. If contracts are complete the vast majority of trades are initiated by public offers. Most trades take place in one-shot transactions and the contracting parties are indifferent with regard to the identity of their trading partner. Moreover, the short side of the market attempts to appropriate the whole gains from trade, which causes much disagreement about contract terms.nIf contracts are incomplete the vast majority of trades are initiated by private offers. The contracting parties form long-term relations and the provision of low effort or bad quality is penalized by the termination of the relationship. The threat of terminating the relation turns out to be an extremely powerful discipline device. "