Hey Crin, sorry I still haven't got that save to you, been having some PC issues this last week. However, before said PC issues came up, I did notice that my Sims cannot actually sell the paintings at the festivals (I may have been mistaken when I thought this was possible, not sure why but the option isn't there anymore regardless). I CAN however sell the paintings painted by Sims on those easels in the Arts Quarter, but only with Sims that actually live in that district. I tested this a week or so ago by having one of my Sims who lives in Willow Creek visit the Arts Quarter - there were completed paintings on the easels but that Sim could do nothing but scrap them.
Sims that live in the Arts Quarter can sell any paintings left behind on those easels there, and only if they are on their home lot at the time (so if they are on a neighbor's lot for example the option isn't there). It seems as though the game sees the outside area as an extension of their lot so they are allowed to sell the paintings.
To reproduce: Create a sim and move them into an apartment in the Art district. Wait for NPC Sims to come by and paint something. As long as they actually finish the painting, your Sim should be able to go down there and sell the paintings.