We have no idea what the in game currency will be based off of. NPC should be used as a way to generate credits always. How else are you going to get credits in the game?
The trading system will most likely take a cut of it so if you put something on the market for 1,000 it might take 100 or so for listing the item. This is the "gold" sink. How are we to generate money if NPC do not generate it? What would an advance society use as money? Energy is something I feel would be a balance.
I could see the credit system be based on "energy" as in if you find radioactive ores, you need those for ship fuel or you can sell to NPC for credits. Then it becomes a balancing of do I need credits or should I use it myself or sell to other players? The Ark ship might also have missions for us to do other things we don't know about yet. We are rebuilding society on this new alien world and do not have a huge amount of systems in place which makes life become very exciting.