The Borsodi Brothers' Auction House is an impressive three-storeys building in Free City of Novigrad, belonging to Borsodi Brothers from the House of Borsodi. It's located on the square at the Main Street, in the most representative part of the city. Auctions were held only once every three months, on Fridays.
In first expansion for the Witcher 3, Vimme Vivaldi tells us that Radovid V of Redania made a deal with Horst Borsodi if Horst moves his auction house near Radovid's kingdom pearl - Oxenfurt. Radovid would then grant Horst a preferential tax rate of zero percent.
The building in Oxenfurt is only two-storeys high compared to the one in the Novigrad, but this one has an underground vault to compensate. The underground vault has a trap plate right after the vault door leading to a pit full of Arachnomorphs, as well as a hidden exit at the end of the vault.