This side quest is available after the auction ends in the part of Open Sesame! quest. To be able to start this quest you must buy the second auction item, Van Rough's painting. You must deliver it to Marcus Hodgson (M1,9), the book seller who has a shop on Novigrad marketplace. For the painting he will offer you 500 crowns and Conserved Shaelmaar Trophy which adds +5% to gold finding. You will also receive some experience.