This is dealing with a low-tech fantasy world, but theoretical applications can go all the way up to your Shadowrun-esque corps as well.
A central commercial hub (big city, whatever this means for your world) is situated on an easily-traversed plain with relatively secure roads.
Lack of water-based access means that no one nation gets advantage in speed or transport; everyone has to cross the same territory to get their goods to market.
The founders of the city were retired adventurers; they had the means to drop initial investment funds into the pot to get the enterprise going and attract the market forces that kept the whole thing going. It helps if there's something financially unique to export from the area. The ruling council was comprised of the adventurers, but over time (as they died of old age or got bored), the people in charge of key contributing industries or trading chapters would join and keep the whole thing going.
Internal politics are evolved from initial founding of the city. As with most plutocracies, the expectation is two-fold: One is obligation, and the other is privilege. The obligation is that to hold the seat, one must contribute significant funds into a pool from which basic services are provided to keep operations running. The privilege is to make decisions that directly benefit you and undercut competitors in the same industries, such that you possess monopolistic power.
If a council member is not able to meet the obligations, the seat is offered for auction (at prices accessible to almost none) and the cycle continues.
What does this mean for everyone in positions of wealth, but not power? There are above-board methods of gaining advantage, such as you might see in typical corporate setups (bribes, shareholder buyouts, market competition) and the shady stuff where PCs might get involved: corporate sabotage, assassination, fomenting labor rebellions and consumer uprisings, etc.
This does not preclude a certain segment of the market to offer specialized/artisan services that are inaccessible through major market leaders, either. If an artisan weaponsmith makes two super-special-awesome +5 swords a year, that hardly keeps her going regardless of markup, while a broad-market weapon producing leader might have a contract to supply all city guards and militias with your basic PHB steel short swords and capture the rest of the market share with ease. If demand for bespoke +5 daggers suddenly springs up dramatically, sure, the artisan crafters might all get buyout offers, but until then it's a wasted effort for no appreciable gain.
What makes this system work and become self-perpetuating? If everyone is selfishly greedy, how would any "greater good" hard decisions get made? Two things come to mind: Either the founders codify delivery of certain public goods/services with the intention of continuing to attract new wealth-seeking population to the city as market losers cycle out, or there's a constant pressure on the city from the outside (e.g.: hordes from the East) that requires investment in things like basic security and infrastructure so that the society is strong enough to resist the threat.
Next up: Thoughts on the seamy crotch of shadowy evil.