Playing my first game and trying to figure out stuff... Who and how can trade systems? I'm playing as UE and got a system from Lumeris. Then I see the option on my end to trade a system with other races but not on theirs. Can't find anything concrete when I search either, besides the Lumeris trading outposts thing.

Thanks.

Hey, pal!

If you want to sell your system to other fraction, you need to open diplomacy hub, choosea fraction, open system menu, choose system, show youe price(sell for dust or other resources), accept trade and... Voila!

Only the Lumeris are capable of trading system during ordinary diplomatic negotiation. Other factions can only trade control of a system when negotiating a truce to end a war. In either case, it works as SakhTyphlosion described.