While I must say I agree that Independent traders are limited nicely, I must say that there is still a way to go towards limiting mercs and freelancers in their trading ability.
A freelancer who needs a large transport to trade with is by definition an Independent trader.
About corporations restricting certain goods I think this is a very acceptable move to make.
Most corporations have something that is their bread winner.
Kishiro has after all signed an agreement for exclusive rights to buy all of the GMGs H-Fuel so they can ship it to others as they see fit.
Now I know that it is normally seen as they are just transporting it for the GMG and I.C. has also got an agreement that they receive 5% of the H-Fuel from the Okinawa system but I am just stating how it is written in the game.
The tricky part about a monopoly is making sure you know who else can still ship your product according to game lore.
For instance with H-Fuel, since I.C. has 5% of the fuel mined from the Okinawa system then of course they may ship it from there.
Likewise Samura has their own mining operations going on and so may ship H-Fuel from Hokkaido.
I am sure that at least Rheinland has one corporation that also mines H-Fuel and although it would not make much profit for them, they can hardly be shot by mercanaries for carrying that product on their ship.
The other thing to think about is that as soon as you start selling lisences for your products you have to expect the corporation at the other end to follow suit to recoup their expenses.