Black Market has two floors. The first one is a small entrance hallway with a variety of friendly monsters that guard the place, all of whom would normally be hostile toward the player. Some will be in 'cages' of iron bars which can be broken open without angering anybody.
The second level is the shop proper, managed by One-eyed Sam. A vast number of items will be found here on an open floor plan though at inflated prices. Sam also has a whopping 30 000 gold budget if you wish to sell some of your stuff.