In the presence of incomplete contracts, implicit contracts may help to overcome inefficiencies. For such an implicit contract to work, however, individuals must interact repeatedly. In our experiment we study whether in a competitive market institution buyers and sellers can coordinate themselves on such endogenous long-term relationships. Our experiment consists of two treatments. In the incomplete contract treatment the quality of the traded good is not contractually enforceable. In the complete contract treatment, quality is enforceable. In both treatments it is the buyers who make offers. However, buyers can choose between two different institutions in order to make their bids. If they offer their bid in a spot-market institution every seller is aware of the specific offer (just as in a typical posted bid market). However, buyers can also choose to make their offers in a bilateral institution. In this case only the seller who gets the offer is informed about it. In both treatments, all buyers and sellers have an ID-number which they keep throughout the experiment. In the presence of incomplete contracts we find that people do in fact coordinate on long-term relations. Within such long-term relations we observe that efficiency increases in the duration of the relationship. Moreover, expectations about the quality choices become more precise and subjects feel more certain about their expectations the longer the relation lasts. Finally, payoffs within a relation are very equally distributed between trading partners. Since most trades take place in ongoing bilateral relations, there are almost no transactions that take place on the spot-market. In the presence of complete contracts the picture is quite reversed. Almost all trades are concluded on the spot-market. No implicit contracts or long-term relations are created. Payoffs are very unequally distributed in favor of the short side of the market