I think random BOFFs and DOFFs tend to be some of the biggest sellers. Knowing what BOFF abilities are in demand, and then putting those on the Exchange, can really net you a tidy sum. Efficient Saurians and BOFFs with untrainable abilities are the big ticket items, IME.
Tour the Galaxy is a great money maker. I make 2 million EC per event (no joke, 50,000 EC Per sector and doing 5-6 laps).
And as mentioned above, either scoring a Galor and selling it (which you are aiming to buy in the first place). Finding a Very Rare DOFF from a recruiting mission or a Ultra Rare Doff from the C-store pack and selling it.
Finding Efficient Saurians (Purple Quality is high demand).
Or scoring a Very Rare MKXII weapon system that isn't from the STFs.
Other than that, the market is flooded and really there is nothing to sell for a good income.