Scalorn

Economies are tricky beasts.
 
Here are some of the NPC based faucets/sinks I can see:
 
a) Buy/Sell Raw Material
Blue Print designs
c) Assembled devices
d) Go scan/explore an area (grid reference, moon, asteroid, solar system).
 
1) NPC Pricing
If pricing is fixed then materials have a floor value.  i.e. if the NPCs always buy iron at 1.3 credits per unit then no player will sell below 1.3 credits per unit.
If they sell for 13 credits per unit no player can sell above 13 credits per unit.. But the farther you are away from the ark ship the less true that becomes.
If you are a 3 hour flight from an NPC then 20 credits per unit might be the best offer you can get.
 
2) Cash availability
If NPC orders are unlimited for buying/selling then you have an uncontrolled economy that can spiral into hyper inflation/deflation.
 
On the other hand if there is a hidden bank account that is the NPC money supply - when it runs out then the NPCs can't buy any more.
This leads to problems with new players starting.
Unless you pair it with an account per player.  i.e. each new player gets 1 million credits in their hidden account.  They can sell that much to NPCs to get initial cash.
When that per player hidden account is empty then the NPCs have to have funds in the global hidden account or they are broken and can't/won't buy.
Maybe selling doesn't refill the personal hidden account or maybe only up to a certain amount.
If the money supply is shown to be too restricted the devs can add more to the hidden global account and the NPCs start buying again.
Maybe it's determined that 100 credits/second should be added to that global account to keep things smooth, etc.
 
3) Bartering
This one would take a bit more work on the devs part to allow the market units to have individual values for bartering materials.
If on your market unit you could say you value iron at 1.5 credits per unit and are selling a particular widget for 15 credits then they could buy the widget with 10 iron units.
This gives lots of flexibility and makes more sense long term to me.  Credits have almost no meaning if I can't turn them into something I want.  And when your dealing with rare/hard to acquire material you will probably want to trade for other materials.
i.e. I want to put a superb engine I built up on the market for 20k credits + 100 units of dark matter crystals.
If you wanted to be really fancy you could support multiple "prices" per item. i.e. 100k credits, or 20k credits + 100 units of dark matter crystals.
Add in the ability to lock those offers to certain people/groups. i.e. alliance members get special pricing.