Energy, biologicals, & ores are basic commodities that are traded on almost every planet. The prices fluctuate from week to week and are determined by a 6d6 roll when arriving in a new system. The highest two are the price of energy in 100’s of credits, the middle two are the price of biologicals, and the lowest two ore.
Many planets produce and export a surplus of one of the three, and at those planets the relevant commodity is available at half normal price.
Regulations prohibit a small trading vessel from carrying more than 10 units of any particular commodity. More can be carried of course, but smuggling is required. One should keep in mind that containers full of energy have a nasty habit of exploding at inopportune times.