ive seen demand for metals ocasinaly, but its rare
ive found trading in drugs, fuel, supplies, and food the only reliably profitable things
one of the odd things i have found is that if you have a massive trade fleet and 3 large planets ~2 days apart
you can buy ALL the food from #1, then sell it to #2 (with food shortage)
then buy all the food from #3, and travel back to sell to #1 (winch now has food shortage)
and cycle the food like that for crazy profit
I got 2million credits cycling 4000 units of food between a few planets like this