It's another main quest after the Evil's Soft First Touches, received at the same time as Dead Man's Party (you can complete them in any order you want). Go to Oxenfurt, to the auction house (M1,3).
Important - you must bring large amounts of money with you. 1000 crowns will be especially useful for buying the diagrams for a collective Serpentine School Gear, and around 500 for winning other items (two) related to the side quests.
Once you get to the banquet room on the first floor, you can talk to three people in order to obtain useful information. Conversation with Yaromir, if you point the proper portrait - it's the one with a townsman - will give you information about the fact that you can buy van Rough's painting at the auction and then sell it with great profit to a merchant in Novigrad (Avid Collector side quest). If you want to earn more on it (buy it for cheaper price), don't tell Vivaldi about it, because he will fight for it during the bidding as well.
Countess Mignole will reveal herself during conversation to be a collector of Witcher gear - you can buy four pieces of Serpentine School gear: gauntlets, armor, trousers and boots from her. The price of the set is around 1000 crowns. You can talk with her more, obtaining some additional experience.
Hilbert is the last of optional talkers. He is a dedicated Gwent player, you can bet with him for 250 crowns if you are sure of your skills. A card (Gaunter O'Dimm) can be won when playing against him as well. Opponent's deck is Nilfgaardian Empire.
After completing the optional activities and getting to the main objective - the auction will start. During the dialogue you can borrow 100 crowns from Vivaldi, but don't tell him that you aren't participating in the auction - if you do, you won't be able to obtain any object.
The order and prices of auction are as follows:
- Count Romilly's Figurine - 300 crowns (it's worth about 150 crowns, related to A Dark Legacy quest),
- Van Rough's painting - 60 crowns (if you haven't told Vivaldi about it - 20 crowns) (worth 150 crowns, related to Avid Collector quest),
- Professor's glasses - 350 crowns (important - Geralt can wear them).
After the auction you can talk with Borsodi. No matter which options you choose, the dialogue will end with you being thrown from the building and forced to fist fight two guards (level 33). After a conversation outside with a stranger, the quest will end. You will be rewarded with experience and, if you bought the painting, the Avid Collector quest will start.
In your next objective you must go to the herbalist near Oxenfurt. Before the hut you will find three guards (level 33) that aren't friendly - they will attack you immediately.
After the conversation with halfling merchant you will be moved to the basement where you will learn the details about subquests required to be completed before you will be able to perform a sophisticated robbery. The subquests focus on recruiting colleagues for the robbery. They are thoroughly described in further chapters.