Gosh, this is an old thread. I should have come back to it! Basically you can only sell paintings that sims other than your own have painted on an easel, if your sim does not need to go through a loading screen from their home lot to get to the easel.
Umm.. did that make sense?
It makes sense, but what about if the Sim does not have to go through a loading screen --- but maybe, you meant that to since the painting is not on the Sim's home lot, then, they have to go through a loading screen to get to Windenburg, etc.
My Sim wanted to get her mother's painting from a lot in Windenburg. It was in front of my Parisian Cafe, but did not require my Sim to go through a loading screen. It was in front of a fountain. She can paint at a level 10, anyway, but I really liked her mother's painting.