You can purchase (free) Space Fuel from any barter merchant at any station where it is marked illegal. The down side, it trades for 0 credits in both directions. But if you take it to a trading dock and use the black market then you can get money for it. (Or hide it and drink later.
j/k) You can sell it at barter merchants in goner space for free credits. Take it to a pirate base. Or sell at the Disruptor Missile and Plasma Burst Generator factories for credits.Edit:
I don't recall this being posted previously. But this is a vanilla game exploit just in case the unofficial patch fixed it.