In Mists of Pandaria, Blizzard is opening a Black Market Auction House. This auction house is bid-only (no buyout) and will feature rare items like mounts and pets. Stuff that is traditionally hard to get, like Rivendare's mount, or Ashes of Al'ar, and requires a lot of farming.
(There's also a possibility that the current tier Heroic raid and PvP gear will show up on the Black Market. As Theck points out, this specifically is a horrific idea. It will cause a lot of issues with Loot as Investment, and I think also devalues those items as Loot as Reward.)
The purpose of the Black Market is to serve as a gold sink for whales. A way for goblins to spend hundreds of thousands of gold without greatly impacting the rest of us. I'm sure that the Black Market will fulfill this goal.
I don't think I have any items that would actually be sold on the Black Market. Maybe an Ulduar or Firelands drake. I certainly do not have enough gold to buy anything off it. So maybe my point of view is skewed.
However, I don't think the Black Market is a good idea. The type of items that would be sold are the type of items that generate stories.
Stories about how it dropped for your guild and you won the roll, or how guild all passed the item to you. Or stories about how you farmed for days, and it finally dropped for you. Or how you worked with a small group of people to get everyone their mount. Or even how you're a lucky punk and you accidentally found and killed the rare spawn while questing without even knowing it dropped a mount.
These are the type of items that bloggers would make blog posts about, and other people would jump in comment about their experiences. Everyone who gets one of these items has some sort of interesting story to tell.
In some respects, the story of how you got the item is more important than the item.
And to all these stories Blizzard is proposing to add, "Actually, I bought it from an NPC on the Black Market."
Ah well, it could worse. It could be a Real Money Auction House.
Edit: I've thought of a parallel situation: the warlock and paladin class mounts. Back in Vanilla, every paladin and warlock had a story about their class mount. There was lots of farming, and you had to rely on friends and guildies. Then Blizzard just gave out the class mount from the trainer, and all those stories dried up. Maybe it was necessary because of the level cap increase, maybe it wasn't. But I think that the Black Market will end up doing for all those rare mounts/pets/items what the trainer mounts did for the paladin and warlock class. And I think the game will be worse off for it.