By no means is this a complete list. This is just a list of what may appear on the BMAH. If you see anything not in the list, feel free to leave a reply telling me what it is, preferably with a SS showing the item on the BMAH. This list was made awhile ago, so there have been without a doubt more items found that aren't listed, so please post them!
Anyways, not my guide, came from WoWpedia (Black Market Auction House - Wowpedia - Your wiki guide to the World of Warcraft) but I searched for about... 5 seconds and didn't find anything.
Oh, and no, I didn't just copy and paste it, I had to go to each individual item on wowhead, get the link, come to OC, insert the link, and then do that about 60 times.
Armor
Tier 3 armor pieces, without the original class restrictions:
Dreamwalker Raiment - Item Set - World of Warcraft
Cryptstalker Armor - Item Set - World of Warcraft
Frostfire Regalia - Item Set - World of Warcraft
Redemption Armor - Item Set - World of Warcraft
Vestments of Faith - Item Set - World of Warcraft
Bonescythe Armor - Item Set - World of Warcraft
The Earthshatterer - Item Set - World of Warcraft
Plagueheart Raiment - Item Set - World of Warcraft
Dreadnaught's Battlegear - Item Set - World of Warcraft
Season 12 belt and bracer pieces
Item level 509 back, neck, ring, waist and bracer pieces
Arcanite Ripper - Item - World of Warcraft
Companions
Argent Tournament pets:
Ammen Vale Lashling - Item - World of Warcraft
Dun Morogh Cub - Item - World of Warcraft
Durotar Scorpion - Item - World of Warcraft
Elwynn Lamb - Item - World of Warcraft
Enchanted Broom - Item - World of Warcraft
Mechanopeep - Item - World of Warcraft
Mulgore Hatchling - Item - World of Warcraft
Sen'jin Fetish - Item - World of Warcraft
Teldrassil Sproutling - Item - World of Warcraft
Tirisfal Batling - Item - World of Warcraft
Shimmering Wyrmling - Item - World of Warcraft
Redemption/TCG pets:
Banana Charm - Item - World of Warcraft
Dragon Kite - Item - World of Warcraft
Hippogryph Hatchling - Item - World of Warcraft
Rocket Chicken - Item - World of Warcraft
Rare drops:
Captured Firefly - Item - World of Warcraft
Dark Whelpling - Item - World of Warcraft
Giant Sewer Rat - Item - World of Warcraft
Gundrak Hatchling - Item - World of Warcraft
Parrot Cage (Green Wing Macaw) - Item - World of Warcraft
Phoenix Hatchling - Item - World of Warcraft
Proto-Drake Whelp - Item - World of Warcraft
Tiny Sporebat - Item - World of Warcraft
Cat Carrier (Black Tabby) - Item - World of Warcraft
Tiny Crimson Whelpling - Item - World of Warcraft
Vendor pets: (note that the bidding price starts at a value vastly more than the actual vendor price)
Cat Carrier (Bombay) - Item - World of Warcraft
Cat Carrier (Siamese) - Item - World of Warcraft
Obsidian Hatchling - Item - World of Warcraft
Mounts
Rare drop mounts:
Ashes of Al'ar - Item - World of Warcraft
Deathcharger's Reins - Item - World of Warcraft
Flametalon of Alysrazor - Item - World of Warcraft
Mimiron's Head - Item - World of Warcraft
Reins of the Blazing Drake - Item - World of Warcraft
Reins of the Blue Proto-Drake - Item - World of Warcraft
Reins of the Drake of the North Wind - Item - World of Warcraft
Reins of the Drake of the South Wind - Item - World of Warcraft
Reins of the Green Proto-Drake - Item - World of Warcraft
Reins of the Onyxian Drake - Item - World of Warcraft
Reins of the Raven Lord - Item - World of Warcraft
Reins of the Vitreous Stone Drake - Item - World of Warcraft
Reins of the White Polar Bear - Item - World of Warcraft
Smoldering Egg of Millagazor - Item - World of Warcraft
Swift White Hawkstrider - Item - World of Warcraft
Redemption/TCG mounts:
Riding Turtle - Item - World of Warcraft
X-51 Nether-Rocket X-TREME - Item - World of Warcraft
Other
600 skill tailoring, blacksmithing and leatherworking recipes
Tabards
Tabard of Flame - Item - World of Warcraft
Vanity Items
Fishing Chair - Item - World of Warcraft
Goblin Gumbo Kettle - Item - World of Warcraft
Goblin Weather Machine - Prototype 01-B - Item - World of Warcraft
Imp in a Ball - Item - World of Warcraft
Paper Flying Machine Kit - Item - World of Warcraft
Picnic Basket - Item - World of Warcraft