It can be difficult at times. I now have a fleet of freighters that run and right now, some are not making any money for some reason. Other times, they really rake in the money. While in a game, I can make nearly a million from the freighters running them selves automatically.
What I do to aquire a used freighter is find one for sale (the sales money signs icons on the stations usually mean they 'might' be selling something). When you find a ship, check it out on that Russian site at the 'Screenshots / Screenshots of Space Ships. Enter the name of the ship for sale and hit enter and it brings up a basic info string on the ship. CLick on that and a picture and tons of detail popup on the craft. At the bottom of the top part is the price they sell for to have them made new. Use that to judge if its a good deal.
Then, when you buy one, get it home safely and have jump drive, Best Buy, Best Sell, Navigation software, Trade Extension, etc, loaded up. Have the engine tuned, cargo bay extended, rudder handling improved. Anything else in software to add that you might need, (trading mk 1, and mk 2), and then send it on its way in a busy sector via 'Sector Trader' in Command Console / Trade section and bing...! You start making money.
The better you outfit your ship, the more money it will start generating. It pays for itself immensely. Now do that with 10 ships and you get a picture of how much money you can 'really' start bringing in..
Finding the abandoned ships really helps too. You can sell those and get freighters and things, unless you find a nice freighter.
And beware, you can find ships for sale that are expensive.. Always check that price list of ships.... and find out if its worth it to get. Some of these ships are old world and now retired, so they dont fly well, etc..