Trade
The Romans considered trading very differently than we do now. It was considered beneath the upper class to be involved with trading. However, trading was a very important part of the Romans and their lifestyle.
At one point, the Empire was about half the size of the United States. Combine this with the fact that it was centered around the Mediterranean Sea and it is easy to see why trade was an important part of Roman life. They brought silk from China, ivory from Africa, and spices from India.
Rome itself produced a lot of cereal grasses, vegetables, fruits (grapes and olives), sheep, pigs, goats that they traded amongst themselves and with other countries. They even exchanged goods such as pottery, clothing such as shoes and mantels, and lamps.
All of these items had to be shipped from one place to another. Often, shipping items by sea was cheaper, easier, and more liked by the Romans. However, during the winter, foul weather would often cause problems for trade.
The every day person in Rome could find some of these items at local markets. These markets were within walking distance to most of those within the Empire. They tended to run, not on a weekly basis, but on a 9-day schedule. People would just line up along streets and set up little areas where they would show off their wares. Those living in the area would come and shop, much like we do today in shopping centres.