The idea is based on regions and not cities.
Your regions gather resources and produce goods. All of your regions production that is not demanded will be free for trade.
This is how it will work. Once a trade route is established between two regions the specific good that are offered will be possible to sell/trade. Every regions population have a certian demand for goods depending on social and technical and economical advances. Certain improvements will also affect the demand for specific goods. If a trade route with the same kind of goods are established from another region the region that are buying goods will first of all buy there goods from the city where it's less expensive. If the demand is big enough it will then buy goods from the more expensive regions. This will happen automatically. Goods that are sold out will slowly become more expensive, goods that don't get bought will slowly decrease in value until your corporations don't want to waste any more money on them. (This is one good reason to have state-owned firms: U may need products that's not profitable).

U can if u want to change the tax rates for different types of goods (but for simplicity not specific goods). U can also forbid, or buy to the government (wich is basically what u do whenever u build something). U can cancel trade routes, or give financial support to certain industries/corporations/markets.

The interface works like this. There are two levels. On the macrolevel (if u don't want too much manegement leave the microlevel to your tradeadvisors)U have a list of which regions has establiched trade routes and how many they have and. And also an indicator on how rich your regions are. Maybe a smileyface when the people have all the goods they demand and a "BNP per capita" number. On the microlevel u can go in to each post/region in this list and see what goods are in for trade. Both import and export. The prize for the goods and the supply and demand for the goods.

U may now wonder why trade is needed. Well
it's beacouse goods that's not demanded will not be sold and therefor it will just go to waste. The trade will significantilly boose up the demand and also the production of your people and therefor it will be important for your economy. One more point in trade is that really big cities couldn't do without them.