I'm not sure why so many people think a real marketplace couldn't work. What would happen is the price of Alloy would go up and up and up, allowing an individual who decided to heavily invest in alloy production to make more and more money. If everyone is buying and you're the only one selling, you're going to make a lot of money. Economies as a rule are not always going to be alloy-capped.
My suggestion is that you introduce some sort of baseline injection of resources into the internal and galactic market so there is always at least a limited supply of goods to buy. Lore explanation: resources produced by the private sector, and/or neutral 3rd party aliens (enclaves, fallen empires, caravaneers, etc). Monthly purchases/sales could be price-locked on a 10 year contract. Monthly buyers would get first dibs on the baseline resources injected by 3rd parties.
I would suggest you guys check out Endless Space 2's Marketplace to see ways they could improve it for Stellaris.