So what's available at the Black Market? A little bit of everything, including rare mounts like the Ashes of Al'ar, pictured above. Also included are pets that require a grind to obtain such as the Sen'jin Fetish, previously available only from grinding out Argent Tournament reputation, and high-level crafting recipes, although we don't have a lot of information on what those recipes are exactly.
However, perhaps the most interesting note to glean from this is that the Black Market Auction House appears to be carrying the original tier 3 sets that were removed from the game when Naxxramas was moved to Northrend and changed from a 40-man raid to a 10- and 25-man raid in Wrath of the Lich King. Obviously, this means that those looking to transmogrify into these rare tier sets can now pay to do so -- provided they've got the gold to fling around.