We could need to check author UUID, i.e. when selling the painting. Checking by nickname will work in 95% cases, but if painting was created a while back, player changed nickname and returned to the game, they won't be able to sell it.

Had an interesting case where moved world from dedicated server to the local machine, and was unable to sell painting due to UUID mismatch. I think that happened because the server worked in an offline mode, and I used Microsoft account locally.