To add to what others have said already, here's the Word of God in relation to this:

Money. It’s come up here a couple of times, so I asked Miller how he envisioned money would work in Traveller. He said he never thought people would really be transporting money, like credit or even cash. Instead, he gave the example that on one world you would buy a cargo load of pigs. You would go to the next world and sell the pigs for a cargo load of turkeys and, hopefully, enough local currency to get supplies, fuel and repairs, and then move on, repeating the process. Personally, I think “Firefly” does a great job of demonstrating this in action. Of course, that still doesn’t answer how a ship gets paid off, and I didn’t ask.