I'm Synth, and I saw profit all the way up the lvls, making purple & orange gear, the lower lvl stuff 9-30 seems to sell the best. Key is to roughly figure out what is cost to make...mats, missions etc.. & sell it for 3-10 times the number. Keep track of what sells & what doesn't and change price accordingly.
I have some orange gear that cost 200 to make & sells for 3k, it's not fast but consistent.
Only run rich missions if you can. Watch the GTN for base price mats & crafting missions...you'd be surprised how often they come up.
Sell you random drop blue gear, GTN base price is usually way to high, but you can cut down the price and still get double to tripple the vendor price.
All of this takes work & time.......lots of credits this way is possible but not easy.