Once more than one settlement or outpost has been established on Mars, inter-settlement commerce becomes a possibility. Settlements can trade excess goods, rather than using valuable space for storage or recycling/wasting the goods. Once stable commerce has been established, some settlements may be able to specialize in a particular area, such as mining, manufacturing, or agriculture.
Trade involving physical goods necessitates a transport infrastructure. Initial transport between young settlements may involve rovers. Larger settlements may be able to invest in a railroad or similar dedicated transportation method.