For Energy Credits:
Hang on to every food item you pick up until you can check their price on the exchange. Some of them, seemingly randomly, are worth quite a bit of money.
All components are easy to sell for a profit, and sometimes regenerators.
Common, Uncommon, and sometimes Rare ground equipment you're usually better off selling to the NPC.
Much of the common space equipment (deflectors, shields, consoles) make you more selling to the NPC, though some stuff goes really well on the Exchange (quantum torpedo launchers, phaser beam arrays, some phaser heavy dual cannons)
Don't forget to sell off new Bridge Officers if you won't be needing them for your crew or for training!
Weapons and Shields Batteries also well pretty well.
Even materials for crafting (data samples and junk) sell for hundreds to thousands each, depending on what it is.
You can get about half of an item's stated value when selling to an NPC, but only a bit more than a third if you recycle it through your replicator.
MORE: The Jem'Hadar stuff, if you got it at a lower level and don't need it anymore, has a huge stated value if you choose to NPC it.
