Isengardtom
Prince
Perhaps you can steal Artifacts in Modern via espionage.You no longer sell specific resources. When you establish a trade route with a city, you acquire whatever resources it produces; same for other players.
Since they are tied to victory conditions and there is no evidence for theming returning (thank heavens), I would guess you cannot trade Codices, Relics, or Artifacts.
Great Dev Diary. The diplomacy system looks very robust and a lot more meaningful and strategic thanks to the influence yield. So glad to see the “faith” gone and this in its place.