I believe that "these goods are unique to this mission" means you're looking for a mission-specific signal source, not just any signal source holding trade data.
Sometimes scanning the nav beacon (drop in and get a scan of the actual beacon) can help (leads to a mission update message giving you a more specific location).
Scan all the signal sources until you find a mission-specific one. It will be really obvious after you scan it (different color, clearly says it's for your mission, etc).
It's also possible that you found a bugged mission. Sometimes that happens, and there's no way to actually complete it.
I'm not sure I've actually done a "trade data" mission, but I have definitely done some "exploration data" missions, so the mechanics should be similar.