You can make some from low level crafting, although not a ton. No real get rich quick scheme I know of, although I will say ground loot is worth far less than space loot either on the exchange or from a vendor. Do more space missions to make a bit more cash upfront.
I made a lot of my credits by selling items on the exchange. Not the exorbitant amounts most people are charging. I usually look for the lowest and price it around there or vendor dropped items or items that are being sold on the exchange for below the price you can replicate it.
Yes, I have seen that. All I do is shake my head and laugh.
You also get credits in pvp. Not a lot but if you sit there for a few hours they will rack up. Did 4 pvp matches before the down time and made around 60k credits. (I am also a tier 5. it may be based on level. Didnt pvp much before then to tell you)