I'm waiting to get the purple treasure unlocking upgrade but I'm not sure what the condition is. I thought it would be number of treasures unlocked but now I have a sneaking suspicion I need to do some number of trades with a specific NPC. I don't know which one it would be though since I haven't been paying enough attention to the text in this game (though I can always go back to read it all)
slight spoilers Afficher plus
the trading system doesn't really scale with your progress in the game so more often than not you could get your reward yourself in the same time it'd take to get the ingredients