Well... right now i'm acting as merc and (i recommend you so) in order to trade, what i do is to...
1. Land my ship on any empty region of any planet.
2. Deploy mi shuttle and start trading with it.
Advantages.
1. Since shuttles are faster, you can make great amounts of profit in a short time (i got 120 millions in 23 hours).
2. With your ship docked you earn...
a. No radine nor pluto spent
b. The fact that your marines will increase their experience safely. Just by keeping them alive.
So, when you reach about 4 or 5 days (dont forget to resupply your ship with nutripacks and to let them rest from time to time) you will have lotsa money and experience to fight whoever goes in your path.
That's the only way to prevent it buddy...
P.D. There is also a useful bug on it... f.e. Lets see you have a trading route between Polaris and Earth. You go from galcom with your shuttle and loads of weapons to Polaris space station. Now, after selling them all fall back to Polaris 2 (the planet). When in planet (shift 9 to planetfall) just "select" your own ship in priority list and "dock" into it.
You will apear in your ship and not in polaris but in earth. Saved half of the time, useful don't you think so?
And finally... does somebody know how to planetfall into specific locations of a determined planet? I mean, it's quite boring to wait until my cruisser reaches 12000 km from one point to another...