The Black Market Auction House, located in the Valley of Four Winds, in Mists of Pandaria, is a place where you can dump some of that extra gold you’ve been accumulating. What it does is put up for bid various rare/epic items, pets, mounts, etc. Items are chosen randomly by the game code and cannot be posted by players. (Click for a slightly bigger view)
You can find it in the veiled stair, slightly uphill and east of the tavern.
Start at the stairs next to the Grassy Cline, Valley of the Four Winds flypoint. Head to the top of the stairs, grab that flypoint, turn east just up the hill from the inn, and browse.
You’re looking for Madame Goya
WoWhead News has some more info on the Black Market AH.