Coming soon with Mists of Pandaria: The Black Market auction house.
Looking for that rare mount? Rare pet? An old tier piece for transmog that you just can't seem to get that one boss to drop?
Keep your eyes on the Black Market.
From the UI, it looks like there's no "buy it now" option. So when those Ashes of Al'ar show up, you don't just have to see it listed - you apparently have to be prepared for the ensuing bidding war.
I don't see how they can do this, really, unless they implement a proxy bid system. To do otherwise would be to invite the unbridled rage of millions of bidders...