I've been playing MMOs for more than a decade and two things have helped me make tons of money in all of them...
Diversity and Patience.
There are three factors to patience:
1) starting from scratch you will not make millions over night. Yes, someone will post here that they made two million credits in their first 24 hours and/or by selling one item. They are the exception not the rule. Initially, chances are the items you craft will generate modest profit margins. But if you make them consistently and persistently your coffers will grow.
Agree with this wholeheartedly. Some fool might find an item underpriced by millions, but that takes a lot of luck.
2) Just because something you have crafted does not sell in the first ten minutes (exaggeration) does not mean it will never sell. You have to accept the fact that you have competition who will undercut you and therefore your item(s) will not sell right away. Don't panic, everything you craft sells eventually.
Well, not everything. you do need to ensure you're making things that are valuable to other players. There are plenty of craftable items that have no real value.
3) There will be times when the pricing of a given item tanks. You need to be willing to hold on to what you have crafted until the value rebounds. And it always does.
Oi. Still waiting for the value of the grade 22 augments and Exotech stims to rebound... You actually need to know when to be in a market and when to get out of it.
#3 of patience leads into diversity. Crafting many different items gives you the opportunity to sell one thing while another remains in storage. This allows you to consistently increase your credits total.
As to what sells. I have an armormech and synthweaver that I use to make and sell blue quality level 49 augments. They cost at most 5k to make and I sell them for 10k to 15k. I will typically craft five each of the four primary stat varieties (usually getting a couple of crits in the process) every day. that's at least 20 augments at minimum 5k net profit each or 100k daily. In reality it is more like 200k daily.
Some good, some bad there.