This is a missable trophy during the quest "Open Sesame!" Bring at least 1,000 crowns. Make sure when given the dialogue choice you choose to bid at the auction. After your wee banker friend gets you into the auction house (cutscene) you will have the optional task of talking to 3 people, then take a seat on the bench when you're ready. Bid on all 3 items until you win them. Very easy.