A good question!
Case 1: Trade Offer Expired
When you don't have a trade computer, or you fly away too long after scanning a station with your trade computer, the offer will go blank, just as you have described.
There are several ways to approach this:
- Order one of your ships to Revisit Known Stations to get the 5-hour subscriptions (you can do this yourself too)
- Drop satellites near stations to get permanent subscriptions of those specific stations
- Buy trade subscription licences from faction representatives to get permanent subscriptions of stations of that faction
Case 2: Normal Economic Behavior
This simply means the station is not buying or selling stuff, hence there is no price and no dots to indicate the size of the trade offer.
Some tips on this situation:
When the station is not selling anything, they are probably experiencing a shortage. Try to determine if they are missing any wares to start production, or (when demand is much larger than supply) wait until the station completes at least one production cycle. Just sell something to them.
When the station is not buying anything, they are probably experiencing a surplus. Just buy something from them, so they will have some space to store new products to keep production going.
Case 3: Unfortunate Events
The station suffered an attack that damaged their storage modules, so they can't store wares. This will make the trade offers appear blank.
IDK how you can help fix this issue though.
