Of course trade was not an easy business. Many city states meant many borders to cross and many tariffs to pay. There were frequent wars and, at times, many pirates and thieves competing for booty on the seas. Inland the problem was tackled with the creation of a road network, which was perfected in the early roman period.
Trade by the sea was more extensive. It was easier and cheaper to carry large quantities of goods by sea than it was by land. Archaeology has shown that a ship in the classic era could carry 20 to 70 tons, but in the Hellenistic era the capacity rose to 120 or more tons. We can estimate that a ship at that time could travel maximum 40 km a day.
In ancient times they traveled mostly in the morning and rarely at night with the guidance of the stars. As mentioned in “The social and economic history of the hellenistic world” by M. Rostovtzef:
“Trading distances became bigger and bigger through the centuries, and relationships among cities grew more and more complex. Athens, bought its wheat from Egypt and the Black Sea. In the year 374 it is known ,based on the collected tax and the price paid that 23.200 tons of cereals were imported to cover the needs of Athens and Peraeus Alexandria in the Hellenistic times traded indirectly with India luxury goods. After all not all things could be trades with nearby cities.
Iron, silver, especially gold, pepper and most notably slaves were to be found deeper in the three continents that surround the Mediterranean. But at the first century BC Rome drew the center of commerce to the west. Slaves, iron and wheat were the main resources, Rome needed on its first days as an empire. Athens, Corinthos and the Greek cities became important once again, as stations for transit trade.”
The usage of coinage played crucial role in trade. Coins were mostly silver, although bronze coins were used mostly in everyday exchanges. Cities cut their own coins, but there were groups of cities that had the same system, so every denomination had the same weight.