The Merciless Mart is the Candor headquarters.
The Merciless Mart is large, at least 18 stories tall and is comprised of a large room lacking windows in which the Candor gather. The corridors are constructed out of black marble and the floors are decorated with either black marble and a white Candor symbol (scales) or vice versa.
Like all faction headquarters it holds small prisons to house trouble makers.
The Merciless Mart was once called "Merchandise Mart", based on the actual place in Chicago. Because only the letters MERC IS MART were left, most people started referring it to "Merciless Mart" because of the Candor's merciless but honest characteristics.
This is where Four was put under Truth Serum, followed by Tris, shortly upon their arrival.