Well let's be honest there's a lot that could be fixed about trading.
Lets start with what others have said actual supply and demand I'll use tea as the example.
just as there is a hidden bulk sale tax the opposite should be true. And I agree that any commodity can be as valuable as gold or palladium if the demand is sufficient. There should not be a limit to the price if people want their tea badly enough they should pay out the nose for it not give a little more than the "galactic average".
But it the rabbit hole goes deeper there a system that I call home that I get a craving to go sims on it and make it profitable home. It's a high tech system with good outfitting. They produce progenitor cells and consumer tech. They will produce a static amount regardless of their demand for materials or manpower. The supply goes to high only if players are not draining the market. In the real world if a producer of materials runs out of supply, they cannot continue to produce their product. On the flip side if they have an over abundance of materials they can produce much more at a cheaper rate. This is not the case in ED, for example again with tea if I want the station to produce more I need to source them with land enrichment systems, crop harvesters, slaves and the like. They produce in access and I switch to exporting the tea and all other crops from the system. Trading these lower class wares should also have some added benefits. Example if a station is suffering a food crisis and you deliver thousands of tones you should not only be rewarded in capital but also in reputation. You already do see a minor boost in trading anything, but this should be a way to quickly increase it.
Another oversight is the black market and smuggling in general. Canisters that are floating in space were not stolen by you they were salvaged. They should not even have to be sold on the BM. Black market prices should remain at half price plus fines if the cargo was scanned and confirmed stolen or illegal. Otherwise it should be at normal rates or higher depending on its rarity within that station. Onion head is a prime example of this. If it is illegal and un obtainable it should be at a 200% markup. Also the black market should not be just a sellers terminal. It's a market which implies buying and selling, experimental tech, military documents, contraband, slavery, rare artifacts and paintings all at exorbitant prices with even higher potential profits if sold at the right place and not detected upon leaving and arriving at your destination.
Lastly before I write a novel, salvage needs to be a profession let's be realistic, recycling would likely be one of the more important things in a closed ecosystem such as a space station. Salvage lasers and recycling separators in ships out to dissect the carrion of fallen battleships. Salvage, scrap, the metals within gold, platinum, computer components ect should be worth closer to the same and allowed as an alternative in profit and activity to regular trading.
Sorry about the wall of text and likely grammatical errors.