You don't need to have each city limited to only produce a single item. Each city can produce most of what it needs to maintain itself. Each city will also likely have some surplus of these basic goods that might be sold to traders to be taken to other cities.
You can still have a very simplified trading economy by having each city have a primary export. You will need to set up your geography to give a reason for such a focus.
Below are some ideas.
The overall territory is very hilly and quite rocky. Cities generally have some nearby farm-able land to support itself, but generally not much more than that. There are a few exceptions.
"Wheat-city": This city is near the mouth of a river. The delta of this river is large and fertile. Yearly (seasonal) floods refresh the soil each year prior to planting, allowing massive amounts of grain to be grown.
"Horse-city": This city is located in a wide, mostly level, valley. Some small streams flow into the valley, but most water is obtained from wells. The rainfall allows for grasses to grow in abundance, but attempts to grow surplus amounts of grains generally results in depleting the soils fertility fairly rapidly. Enough fields are maintained through liberal use of horse droppings to feed the city, but not much more. Large herds of horses are maintained on the grasslands in the valley. A reason is needed to prevent cattle from also becoming an export. Perhaps cattle ranchers are looked down on in this culture, or that the horse quality and prices are so good, that using any of the limited valley floor for any herds but horse-herds is simply not done, or only done on a scale small enough that anything produced is consumed locally.
"Marble-city": This one is easy. This city has the only substantial marble quarry or quarries. A reason will be needed to prevent this city from also being an exporter of sculpture. Perhaps the city is dusty and seen as uncultured, so that artists are not attracted to the place. Or perhaps this city grew from stone-cutter camps that were initially set-up to provide the stone for an existing city with a large population of sculptors. Even as this city grew, the sculptor guilds were still in "Sculpture-city".
"Lumber-city": This just requires that most trees are simply not suitable to turn into lumber. One or two forests may have prime lumber trees and each have a city nearby to harvest it.
"Ore-city": Also easy. The type of ore is only available at these locations. "Silver-city", "Copper-city", "Iron-city", etc.
"Tool-city": Perhaps the method of forging quality steel, is limited to a single guild that jealously guards its secrets. Or perhaps the local ore quality and mixture is such that the steel is just naturally superior. The local ore would never be sold to outsiders, but would be consumed internally to produce the worlds best tools, armor and weapons.
"Fish-city": This city is located in a narrow valley next to a swiftly flowing river. Yearly salmon runs results in massive catches. In the off part of the year, boats and nets are used to fish out beyond the mouth of the river to catch other types of fish.
"Wine-city": This city has the only climate and land to grow the ideal grape for their wine or hobbs for their beer. Other cities may make their own, but most will be consumed locally. Only this city produces enough in the bulk and quality that results in the demand for the export.
"Cattle-city": Another wide valley that is useful for herds. The culture here is that cattle-is-king. Some horse herds are maintained, but only to keep up with the needs of the local "cowboys".
"Slave-city": This city is on the coast of a relatively undeveloped continent. The local tribes had a culture where taking a defeated enemy as a personal servants was common. When outsiders set up their city on the coast, selling these servants became an easy way to make money, which resulted in tribes going to war with each other to capture more slaves to sell.