Trading at NPC stations is virtually impossible (DS)
When playing on a server, with multiple player factions, with multiple players in the faction, it is virtually impossible to trade at the NPC stations.
The reason for this is that to be able to trade, from your docked ship's inventory, you need to own ALL of the blocks on the ship, otherwise it doesn't show up as an option when trying to fulfill buy orders, complete contracts, etc.
Ideally, the ownership requirement would be that all the blocks on the ship are owned by players in the same faction.
Current workaround is to put a landing gear on your ship, build a separate "ship" that has a cargo container, a connector and a battery. Grab that rig with your ship's landing gear and take it to the station. Once there, dock THAT mini-ship/cargo module to the station and you can trade successfully.
This issue comes up because sometimes the trade/contract requires more items than you can carry in your personal inventory, and contracts, specifically, do not allow you to partially fulfill them.
That's another way to fix this, if the other suggestion above is more difficult: allow contracts to be fulfilled partially. (i.e. Contract for 1000 bulletproof glass, grab as many as I can in my personal inventory, like 300, and fulfill the contract, go back to my ship, grab another 300, rinse and repeat until I've given the contract the full 1000)