SPOILERS AHEAD! YOU’VE BEEN WARNED!
Open Sesame is one of The Witcher 3: Hearts of Stone’s main quests and the second of Olgierd von Everec’s wishes. Olgierd requested the house of Maximillian Borsodi but gave no more details. This is a rather confusing request to Geralt, since he has no idea how to transfer a house to another place. The only thing he does know is that he has to visit an auction house in Oxenfurt.
The auction house is owned by Horst Borsodi, the son of the late Maximillian. Geralt will need to speak with this man in order to continue the quest, but that isn’t so easy. Getting into the auction house is no easy matter for common folk, but luckily, Master Vivaldi (owner of Vivaldi Bank) is around and he happens to be one of your pals. He gets you in and informs you that you will have an opportunity to speak with Horst after the first round of auctions. As you wait, Vivaldi will point out three individuals to you that you can either interact with or completely ignore.
Make sure to speak to Countess Mignole, since she is your only opportunity to obtain a rare set of armor, and Yaromir for a chance at a nice turn of profits. Again, you can completely ignore these individuals, but you’d be doing yourself a disservice. Once you’re ready, you can sit on the bench to start the auction, or it will automatically begin after you’ve talked to all three of the other important guests. You don’t need to purchase anything during the auction if you don’t want to. Afterwards, there will be an intermission where you’ll get a chance to speak to Horst himself.
No matter what dialogue options you choose here, you’ll be thrown out. It turns out that Mr. Borsodi didn’t like your line of questioning, even if you were being polite. Once outside, the guards will try to beat you up, give them a sound thrashing and go about your merry way. It is now that you will meet The Stranger and the Open Sesame becomes a much different quest than you were probably expecting.