Difference Between Collector and Gallery?

The "Sell to Art Gallery" interaction is available for sims in the Painter career for any quality of painting. As said above, if the sim is not in the painter career, only masterpieces can be sold to the art gallery and all other qualities to the collector. According to Carl's Sims 4 Painter Career guide, the Art Gallery will buy a painting for 20% more than its face value.

I think it may be that way in RL. A collector may even pay more than a gallery. You'd have to check it out. Make a copy of the painting and sell one to the gallery and then the other to the collector.

Unless something was changed you can sell masterpieces to the gallery they are worth more this option should pop up after the notification that you completed a masterpiece. Everything else goes to the collector at least since the last time I had a sim paint a lot of my sims do though.