When you're trading you have to maintain a savings or your game is pretty much over if you get killed.I wish I could set a mimumum / have a savings account with the Federal Galactic Reserve. I myself almost went below my minimum when I wasn't paying attention and splurging on some upgrades.
I actually filled my cargo hold once and didn't notice how much I was spending. I ran the route with 200 credits to my name and a rebuy price of well over 20,000 credits.
This on a night I'd been interdicted a few times and nearly bested once.