That's a good point about the fantasy trope of a "trade city"... in reality, it would be more accurate to have a "trade quarter" or a "market district" in every single city... while certain areas may have specific local resources they produce, and other areas may simply be well-positioned at the intersection of several trade routes (eg, river deltas), *all* places would be engaging in trade in a similar degree, once you get above the size of a village then basically nowhere is going to be COMPLETELY self sufficient without commerce
I veiw the "trade city" label in the same way we think about financial institutions. Sure, all cities engage in trade, just like all businessed engage in finance, but some cities' primary economic activity is to facilitate trade between other locations, usually by blocking a trade route and demanding tribute to pass.
That's the thing. Pretty much all cities did this. You had to pay a toll to enter the city, use its roads, and they usually had a market and other forms of economic activity you probably want to exploit if you're passing it as a trader.
When I write around trade, my first look is at the natural and produced resources, then how those color or are colored by the culture, and how the abundance of this resource at the source changes what things are most desirable. Then its just a matter of "Well this passes along x many population centers, so its uncommon or rare beyond this point", and kinda leave it at that.
My favourite fantasy magical resource is Suspendiun from the strategy game Airships: Control The Skies. Basically it's a fantasy gemstone that is naturally buoyant in air so stick it in a vehicle and bam, you can fly now! Very convenient to insert into your story if you say that it's naturally formed deep underground and has only become available due to deep mining, a geological event or some such!
The thing that I use that is closer to trade cities than what you're talking about is what I call league cities. The hentaitic league is a sor seventh variation for what I mean by a league. The lead cities are home to a league which is sort of a wholesale hub of an allied set of merchants. So there will always be a reserve price coming out of the league for league merchants and if the local price becomes significantly off of significantly off of that, the local league merchant will set up a rebalancing supply line until the supplies/prices rebalance. This means that the long distance goods travel in know hubbins spoke network like that of the traditional airlines. This also means that there's a large set of goods that you can get anywhere at something resembling similar prices and similarly to sell on the local merchant of whichever legue. Does this system fall under your objection to the concept of trade cities? It is definitely a city that has a lot more trade than it's normal (regional trade hab) explanation would explain and therefore a higher population and wealth.
In my world there is a local ruleing family and its cadet branch which controls a moderat tradecenter. But the hidden conflict lies in the illegal trade of prisoners, orphans etc. (Anyone who wont be missed) between that ruleing family and the slave empier in my world. Slaves are always in demand and no one can decline a good amount of money.
Slave trade is always a good source of conflict. Especially for the modern reader or player who (hopefully) has negative strong opinions about it right away.
Barter economies are actually a myth. No really! Adam Smith speculated their existence and coming from a world with money it seems intuitive, but there basically hasn't ever been a society that functioned like that. Instead we had something similar but different: gift economies. In gift economies (which were probably the first economies) things actually worked on a credit system. You give someone something without an immediate trade, instead, people pay you back for goods and services in the future. This system only works in small communities where everyone can keep track of each other so as agriculture made larger communities states, religion and even the first writing developed to more strictly track people's credit- money came out of this mostly so states could pay their armies.
Gift economies still exist today, but they are a form of trade without the need of markets. Barter happens whenever there's no money but still some kind of market economy, like in the early middle ages for example, or in other places whenever carrying money around just isn't the way to go. When a merchant sold something within a city, especially when it's sold to a larger guild or other organization, it's has been in exchange for wares. The credit system within the city, between the many merchants and merchant families on the other hand kinda functioned like the Gift economies again which is a bit funny, considering everyone had debts with everyone.
