i) the agent of the seller, or if the seller is not using an agent
ii) the agent of the buyer, or if the buyer is not using an agent
iii) the buyer, provided the seller, agent or buyer is an art market professional.
In practice, the art market professional may provide, in their terms, for the buyer to pay the Artist’s Resale Right (ARR) royalties.
You can find out how the Artist’s Resale Right is calculated via the ACS Calculator.