Trade terminals and the game itself are buggy:
Here are some things to be aware of when operating Trade Terminals (or Kiosks):
When operating trade kiosks and the “Sell” tab is currently selected, the kiosk screen will continue to show the “Sell” tab as active but it will be populated with the “Buy” commodities if you back away from the kiosk or time out and subsequently re-engage the kiosk again. The fix for this is to click on the “Buy” tab, then click on the “Sell” tab again.
If things continue to not work, try a different server, then try a different ship.
There are issues when selling large quantities of goods. I am pretty sure this has to do with an upper limit on what a given landing zone will buy from you. Rather than a good error message, it just throws undefined. To deal with this, try first to sell everything, if you get undefined, then try selling less and less until a transaction goes through, then close the terminal and wait 60 seconds before trying another sell of approximately the same amount.
Occasionally servers will just get screwed or your presence within them is screwed, this can cause nothing to be buyable/sellable no matter where you are. If this happens, fly to PO/Loreville/Levski and logout/log back in.
Sometimes what you actually have in your cargo and what the terminal shows in your cargo are not the same, this will result in undefined errors. Close terminal and reopen it, toggle buy, then toggle sell, then select your ship. Make sure you don't see "undefined" anywhere on the terminal, if you do, close and open it again.