You can manage your carrier's finances by setting tariffs and adjusting the buying and selling prices for commodities traded in it's market.
If this market exists independent of the pricing structure of the BGS, this has a huge potential for gold selling.
How could this be managed to prevent this?
If i had to hazard a guess. (which i am normally wrong about these things)
Maybe Carrier import and export volume and price is determined by the natural resources of the system it is parked in.
System with many ringed planets may naturally export mining material and equipment, but demand food and basic living items.
The Carrier owner can then set a tariff on the primary export item as the BGS wants to buy these items quickly due to the low price and large quantity. This tax then brings in extra credits to pocket or support the carrier upkeep cost.
In addition the Carrier owner can make a % adjustment to the galactic average price of any item sold or bought on the carrier. perhaps plus or minus up to 10%.
Giving the Carrier a way to (undercut) any competitors in the same system.
Perhaps the carrier is parked really close to the primary star, so they can afford to increase prices due to short travel distance, or the flip side they are really far away and have to drop prices to lure in customers to travel the distance for a better deal.