In this dissertation, I analyze the formal and informal methods by which cross-cultural commercial interactions, within and beyond the Greek world, were promoted from the fourth to the first century BCE. I begin with Athens to demonstrate the various legal institutions that regulate economic activity between various actors and how they exclude or include foreigners who come to their ports for trade. I then broaden my analysis to Hellenistic poleis and the ways in which they replicate these institutions or innovate new methods to ease the transaction costs of long-distance, inter-state trade, even under the imperial control of Rome. My focus shifts in the last chapters to include Near Eastern, North African, and Arabian polities, their economic similarities and differences from Greek poleis, and the inclusion of their citizens into the Greek milieu. In particular, I show how non-Greek foreigners found ways to imitate or adopt Greek patterns of interaction in order to facilitate their own commercial needs and desires. My final chapter is a case study of the Nabataean Kingdom and their interactions with the Greco-Roman world from the third century BCE to the first century CE. Throughout the work, I blend New Institutional Economics and social networking theories in order to best explain the multifarious connections between merchants, traders, ports, markets, and states across the Mediterranean.