I'm going to preface this by saying that some people are going to think that this is common sense, some people are also going to say that you can just sell them on the neutral AH (where applicable). If you don't like the idea behind it then don't worry about it and please just move on. If not, then read on!
Good website to use to check current/past prices and see if any of this is even worth doing: wowecon.com - World of Warcraft Auction House Price Database
I would recommend using that website before you go and purchase the more expensive items, the smaller 50s items and the like are guaranteed profit but the more expensive items are usually a lot riskier because there may not even be a market for them.
Horde
Pets to Purchase from vendors They all cost 50s except where noted (best to purchase multiples, I usually buy at least 10 of each):
Black Kingsnake - Item - World of Warcraft
Prairie Dog Whistle - Item - World of Warcraft
Golden Dragonhawk Hatchling - Item - World of Warcraft
Silver Dragonhawk Hatchling - Item - World of Warcraft
Pet to farm (this sells REALLY well, but its a rare drop, if you plan on changing but not til a later date its a good idea to just camp your toon there):
Pets to check the AH for: (Buying these cheap on your side and taking them to the alliance for a quick profit is an easy amount of money *if you have the money*, anything 400g or less is pretty much a guaranteed buy)
I can usually buy these for 400 or so, flip them for over 800. But, thats entirely dependent on your server (as is a lot of methods).
Durotar Scorpion - Item - World of Warcraft
Mulgore Hatchling - Item - World of Warcraft
Sen'jin Fetish - Item - World of Warcraft
Tirisfal Batling - Item - World of Warcraft
Enchanted Broom - Item - World of Warcraft
Recipes to purchase (these don't sell as well as pets, but there's also less people selling them):
Only linking the 3 that I have personally had success selling, I don't know why these sell better then the others...
Recipe: Scorpid Surprise - Item - World of Warcraft
Recipe: Bat Bites - Item - World of Warcraft
Recipe: Lynx Steak - Item - World of Warcraft
Alliance
Pets to Purchase from vendors They all cost 50s except where noted (best to purchase multiples, I usually buy at least 10 of each):
Cat Carrier (Bombay) - Item - World of Warcraft** This pet is sold by Lil Timmy - NPC - World of Warcraft as many of you know, it fetches a very high price on the horde AH and its a great sale if you can manage to get one from Timmy himself. If not, its usually still worth it to buy from the alliance AH for 100g or so and then quickly resell it on the horde AH for 400g+.
Cat Carrier (Silver Tabby) - Item - World of Warcraft
** Cat Carrier (White Kitten) - Item - World of Warcraft
Rabbit Crate (Snowshoe) - Item - World of Warcraft
Great Horned Owl - Item - World of Warcraft
Hawk Owl - Item - World of Warcraft
Blue Moth Egg - Item - World of Warcraft
White Moth Egg - Item - World of Warcraft
Yellow Moth Egg - Item - World of Warcraft
Pets to check the AH for: (Buying these cheap on your side and taking them to the horde for a quick profit is an easy amount of money *if you have the money*, anything 400g or less is pretty much a guaranteed buy)
**Cat Carrier (White Kitten) - Item - World of Warcraft (Mentioned above)
Ammen Vale Lashling - Item - World of Warcraft
Dun Morogh Cub - Item - World of Warcraft
Elwynn Lamb - Item - World of Warcraft
Mechanopeep - Item - World of Warcraft
Teldrassil Sproutling - Item - World of Warcraft
Recipes to purchase
Only these two have ever really sold:
Recipe: Beer Basted Boar Ribs - Item - World of Warcraft
Recipe: Fillet of Frenzy - Item - World of Warcraft
Neutral Stuff
These items may or may not be able to flip. However, I've had success both ways when looking at the more noticeable items. For example, on my server the Frosthide Leg Armor - Item - World of Warcraft cost 140g or so alliance, on horde, at least 250g, usually 300g and you can check wowecon to find out. So, without much ado:
Frosthide Leg Armor - Item - World of WarcraftAny of the major glyphs, the difference on my server is usually 20-30g either way. As such, its usually worth buying up some of the 3-4g glyphs and then changing and getting at least 10g per, usually more.
Icescale Leg Armor - Item - World of Warcraft
Eternal Belt Buckle - Item - World of Warcraft
Titanium Rod - Item - World of Warcraft
Titanium Plating - Item - World of Warcraft
Hope you enjoyed it, it probably could of been more thorough but I didnt have much time! And sorry the format kinda sucks. I'll try to fix it a bit.