We use field experiments in Niger to investigate the nature and efficiency of contractual structures in market transactions between kossai vendors and cowpea grinders (key input suppliers). Three contractual structures were employed: gift contract, standard price contract and discretionary bonus contract (most incomplete). Gift contracts and standard price contracts involve an upfront payment of grinding fees where discretionary bonus contracts involve payment after the quality of service is observed. Gift contracts were found to be the most ex-ante efficient with the highest acceptance rates. Discretionary bonus contracts (most incomplete) were the most ex-post efficient that is, resulted in the highest quality. Our results suggest that the degree of incompleteness of different contractual structures influences the outcome of market transactions in the cowpea street food sector in West Africa.