Who is paid when the item moves? It would seem that the artist set up his own eBay account and would be paid every time it was sold. If this is the case, why do

Who is paid when the item moves? It would seem that the artist set up his own eBay account and would be paid every time it was sold. If this is the case, why does he want 15% of the appreciation between sales? That would only make sense if the recipient was paid by the new buyer.