Blizzard shut down the Auction House on US realms and will be turning it back on very soon, as there were some shenanigans going on with a tradable PvP upgrade currency. Malicia, the PvP vendor, will remain disabled for a while yet. 

AH (Source)

We expect to have the Auction House re-opened in the next few minutes.

In Valdrakken, Malicia remains disabled, as we work on this issue and work to restore competitive fairness. The PvP upgrade currency (Trophy that was tradeable) is taking a little while to clean up and preserve competitive fairness.