Letter from Madam Goya
If you complete the quest  ω τ ϖ in The Veiled Stair, Madam Goya will send you a letter in the mail thanking you and informing you about the Black Market Auction House (which is south southeast of the Tavern in the Mists).
This section concerns content exclusive to Warlords of Draenor.
Goya has been removed from Pandaria and replaced with "Retired Exchange Guard". He states, "Madam Goya has decided to move her operations to more lucrative markets. It seemed like a prudent time to give up a thankless job."
She can now be found near the Draenor version of the Ring of Blood in Nagrand.