After making a ton of gold with this today, I thought I would share with you fine folk. All you gotta do is clear raids, and sell pets.
What you need-
A level 90
Time (about 3 hours)
Ok this is really simple, all you gotta do is go to AQ40, Naxxramas (I'd recommend 10 man, as it is easier to solo), Molten Core, Blackwing Lair and clear them.
Easy as pie.
Here are the items you are looking for-
http://www.wowhead.com/item=93040 - Twin Emperors, AQ40
http://www.wowhead.com/item=93041 - The Prophet Skeram, AQ40
http://www.wowhead.com/item=93039 - Viscidus, AQ40 (WARNING: To kill Viscidus, you must have a class that can deal frost damage, or a weapon that can deal frost damage)
http://www.wowhead.com/item=93035 - Golemagg, Molten Core
http://www.wowhead.com/item=93034 - Magmadar, Molten Core
http://www.wowhead.com/item=93033 - Sulfuron Harbinger, Molten Core
http://www.wowhead.com/item=93038 - Chromeaggus, Blackwing Lair
http://www.wowhead.com/item=93037 - Broodlord Lashlayer, Blackwing Lair
http://www.wowhead.com/item=93036 - Razorgore the Untamed, Blackwing Lair (Most likely need 2 people for this, one for the orb and one for killing the adds)
http://www.wowhead.com/item=93032 - Loatheb, Naxxramas
http://www.wowhead.com/item=93030 - Maexxna, Naxxramas
http://www.wowhead.com/item=93029 - Gluth, Naxxramas
I find that the ones from AQ40 and Naxxramas go for the most money, but it highly depends on your economy. Just this morning I made 3800G from the Viscidus Globule. Prices of pets can easily be doubled if you have the time and effort to level them to 25, but I don't.
That about covers it, got any questions, just ask.
NOTE: To sell the pets you must first learn them, find them in your pet journal, right click them, then put them in a cage.
PLUS in 5.3 three old TBC raids will be dropping rare battle pets as well, that means more money!