Question: How is this really any different from what is currently implemented?
Just how different would depend on the game mechanics of the the new trading system, for example.
You do not trade directly with any AI, you buy and sell directly to the market.
You do not know who is supplying the market with specific resources, or who is purchasing them.
If resources are limited, eg you discover iron, and it represents 100 "tonnes" of iron for 20 turns (total 2000), you could decide to sell to the market 20 tonnes for 10 turns. You are guaranteed a base price from City State trading, but you may get a higher rate if AI Civs are buying. You could trade 10 tonnes for 50 turns (500), a trade which lasts 30 turns past the mines lifetime, if your stockpile from the mine is used up on your own building programme before the contract with the market runs out you could suffer a large financial penalty and reduced trading rates for subsequent deals.
There could be limited market availability of strategic resouces, so that even if you don't have your own supply of oil, you could be guaranteed to be able to buy sufficient to build 2 destoyers.
There are opportuinites for a massive expansion of corporations into the market.
The options are no more limited than the real global markets, buying and selling of arms, loans, trading debts, trading futures.......
Just imagine you are at war with Monty, he has taken out a huge loan with Hatty to fund the war effort, you buy the debt from Hatty, and pull the rug out from under Monty's feet.