Its a blue waistcoat with a bow tie
The Auction Staff outfit appears in the following:
Inside the library on the second level, which is the most western room on the floor, there is an Auction Staff NPC alone in the room. You can subdue him and hide his body in a body container in the room. Keep in mind that various actions you take during a mission might trigger some NPC's walking cycles to change, meaning some characters could potentially walk through the room. Also note that some doors of this room are automatically open but no one should notice you taking down the Auction Staff member anyway.