Final answer:
A Persian in Susa could use the road network, such as the Royal Road, to trade or sell goods with someone from Sardis, facilitated by the empire's infrastructure and communication system.
Step-by-step explanation:
A Persian living in Susa could use the new road to engage in long-distance trade with someone from Sardis. Thanks to the infrastructure of royal roads, inns, resting places, and waystations with stables for horses established during the Persian Empire, travel and trade became more feasible and secure. With the system of mounted royal messengers, communication was much faster, enabling trade of various goods to flourish. Goods and messages could be transported rapidly across the empire's extensive network, including the famous Royal Road, which played a crucial role in connecting different parts of the empire for administrative and commercial purposes. Therefore, a Persian could trade commodities or manufactured goods, such as textiles or metalwares, with counterparts in Sardis, and vice versa, boosting economic prosperity and cultural exchanges within the vast empire.