[detaro]

The right they contractually set with the buyer. Secondary market prices exploding compared to first sale prices isn't exactly uncommon in the art world, so why artists would be interested in this is fairly obvious. (To a degree it can also help keep first sale prices lower, it somewhat discourages quick speculation resales (which many in the art world think is a good thing), and lets the artist participate in later value growth of their work which given that the value is due to their work many people would say is fair, and thus they agree to such contracts)