Advancement in rank has nothing to do with the amount of credits to your name. It is releated to the number of missions you complete and ships destroyed. The only way to earn a lot of credits is to haggle on the trade-net. By staying in the space station on the net, and trading in guns, food and gold, you will quickly build a fortune. Travelling to space stations requires you to jump until you cannot get any further.
Load the game. Enter the net. Type "HELP", and get yourself an ID number. Logoff the net. Press the 'BACKSPACE' at the bay doors. Blast off into space, and find a spaceship for you to trade with and get his ID number. Enter the net, logon, and press 'T' and 'RETURN'. You should be now in communication mode with the alien. Say hello, have a chat, keep pressing a key or whatever until he offers you some goods, such as Hypermint for example. At the prompt, straight after he has offered you something to buy, enter "1" and press 'RETURN'. He will say something like no chance, or sign off. Logoff.
Now everything you buy or sell, you will buy or sell it for 1 credit, even if you put 100 credits as an offer, you will still buy or sell it for 1 credit. This only applies to Black Market trading, it will not work at a planet or space station.
Do same as above but enter a higher number, say "20000" when you want to sell at a high price. Now, press 'T' and 'RETURN' to talk to the alien, make certain that you have got something to sell. Keep pestering him, until he says something like, "Have you got anything to trade?". When ask for price enter something, but you get the above set credits
The top routine, is only to be used in buying, as it is no good trying to buy something, when it is set at 200000? Likewise, the bottom routine is only used for selling, as you are a bit stupid if you try to sell your wares for 1 credit!