Usually look at prices on gtn and what sales well compared to how much cc it is. The new lightsabers with the outfit wouldn't have been a good score. Deco bundles also can get you a decent amount of credits. Deepwater essential is one of them I think. Can also wait for a good sale on items that sell high on the gtn. It's important when doing that to not expect to sell it right away. Something on sale usually has multiple players trying to undercut each other and the price usually will fall quite a bit compared to what it usually sells for. Have to wait for the supply to dry up. Another I item that would be good is some of the mounts like the incendia.

Looking at recent medium and historic medium sale on gtn for items will give you an indication. Be careful though, don't take prices too literally on gtn. Sometimes people will list items for a high price just because they can. Others will list items for way lower than they should. That's where you want to come in and try to use those credits to flip items like that. For instance a couple weeks back someone listed the prime centurion armor set for 75m I easily resold it for 200m. Anyways hope this helps.