Hint, NPC's do not buy and sell goods through the market. The price changes of the market is done through computer algorithms, goods sold on the market by the players do not appear in the sale column of said market, only goods purchased by players reduce the number of goods available in the market. If you apply this to player FC's you will end up broke in a few days because the goods sold to your FC market will just vanish even though you have paid for them. You can only have so much goods on the FC to sell, whereas station markets sometimes have stock in the range of hundreds of thousands for multiple items so you can never make enough money selling stuff to cover the loss of buying stuff! On the other hand if you make it so goods sold to FC's appear on the FC market then you will run out of space in a day and will unable to buy and sell anything, again because of the space limitations of the FC. In every way the market system as it works in stations won't work for FC's because FC's aren't station markets.
I may have written it too complicated.
In the case of this proposal, your market will be transformed into a market exactly like the one at the station. You don't care what happens there, you just get a percentage if people do it.
It's hard for me to explain it any other way. Here's an idea, though.
When the player or you yourself, will be in the market menu you will not be able to understand how you are on the FC or on the station.
It's like a gas station service.
This market has nothing to do with a place on your FC. It is not allowed to sell what is in the hold of your FC.
For example, I have 500 tons of diamonds. I want to sell them and I fly to my FC and sell there on the market. I don't have to fly to the station next to me.
Another option. I have 5k diamonds in the hold of my FC, I dig up. I want to sell them in such and such system. I jump on my FC to this system and there just sell my 5K diamonds on the market in the FC. I don't have to take them to the station next to me.