Well, I think the devs are VERY close to a trading system. Seriously, Elite Dangerous just uses a list of objects to buy, they never show the goods. You get a sound effect when the cargo is aboard your ship! ‘Chunk!’ and another when it’s taken off… ‘chunk!’.
But I’d probably go with a passenger type missions. Just have a popup window with available contracts. "Deliver xxxx to xxxx station within 5 minutes for xxxx credits. When the player arrived inside a hit box on the next station, show a success/fail dialog box. You should be able to program this really easy and quick?
I understand this would splinter the fighting force and many pilots would simply opt for the passenger mission, but I think you would gather far more players with this really simple system. (Ghost mode credits = one tenth) It might be tough to leave or enter a hostile station under fire!