Sell from Stockpile to Upgrade/Repair, Sell Minimum Quantities Only
Sell Everything Under Shortage Rumor Only
Harvest & Stockpile Only During Resource Rush Rumors
Operate Out of "Trade Embargo" and "Economic Boom" Rumor Planets (Buy Fuel, Recruit)
Operate Out of "Spice Trader" Rumor Planets (Morale)
Get a 100+ Cargo Hold
Avoid Trade Resources that Require Trade Permits
With a 100+ hull you can easily stash thousands of points of harvested water-fuel. Once that is handled the Captain can move on to finding goods (Harvest for Merchant, Explorer for Explorer) and stockpiling them for sale.
Harvesting can be a good survival technique and the sales of harvested or discovered resources can sometimes offset the Captain's losses . However, large profits will not be realized until the Captain masters the Spice Hall, Rumor Page and Resource Reports.
The Captain that knows he won't sell metal for $500 here because the going price is $5,400 on the next planet over does not cover his expenses -- that Captain gets riiiiiiiiich.