Disallowing users to sell inventory for free: Idk if it's just Arron the Druid Merchant who allows this, but in the Barter Screen, if you put items to sell and press the Symbols of Justice image before you press "Barter," you have just given away your stuff for free.

Original Sin 2 didn't allow you to do the above which was great. I know BG3 doesn't have to be like OS 2, but maybe a warning pop-up would help fools like me that make mistakes?