There are plenty of guides to trading Gems/Ores/Epic Recipes from one PVP faction to another, however all of the servers I am on have identical pricing on each others' Auction Houses. So, those Gold Making tricks have never worked for me.
Here's a list of what DOES work however:
1. [Recipe: Kaldorei Spider Kabob]
I create a new Night Elf toon and quickly grind it to Level 2. I run to Dolanaar, and mail myself 7 Small Spider Legs from another toon. I accept the quest and then turn it in for the Recipe. Transfer it through the Neutral AH to the Horde side and put on the Horde Auction House for 300G. This sells atleast once a week and I've never tried advertising it.
(Note: Use a random number such as 313G 87S 23C so that the more knowledgeable deal-seekers will think it was automatically generated by Auctioneer and hence actually worth that value.)
I'm guessing it will sell even better once WOTLK approaches. There are a lot of folks starting to search for rare cooking recipes for the new Achievement system. Most NE toons just learn this recipe when they complete the quest and forget about it, so you won't see if often on either AH.
2. Other Faction-Specific Cooking Recipes like [Recipe: Westfall Stew]
Here's a link: Faction exclusive recipes - WoWWiki - Your guide to the World of Warcraft
The ones I can buy from vendors I put on the opposite faction AH for 15-20G each. I typically sell 3-5 a daily.
People on either side usually find these by accident. So, if you want to sell major quantities and make a ton of gold, make a public announcements such as "Get your Achievements Now! Selling all Alliance-only Recipes for 5-10G Each!"
3. [Pet: Black Kingsnake]
One of my favourites. I sell these for 40-50G each on the Alliance side, and can purchase an unlimited number in Ogrimmar. The other snakes don't sell very well, I think it's the name of the pet that throws them off.I sell 2-3 daily.
4. [Formula: Enchant Chest - Lesser Mana]
Only available on Horde side. I sell 2-3 daily for 38.5G each.
5. [Schematic: Red Firework] & [Schematic: Blue Firework]
Sell 1-2 daily, I sell them for 25-35G each.
6. [Recipe: Savory Deviate Delight]
My other favourite, and obvious to most people on MMOwned. I buy these on Horde AH for 50-100G each, then sell on the Alliance AH for 300G. I sell 1-2 daily easy.
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How about the rest of you? Any other ideas you have for trading across faction AH's? Gems/potions/etc never work for me, they're always priced the same. Can you think of any other items that are very rare on one side and always sell well? (I'm looking for "sure things" like the items listed above.)
(Note: I have no idea if this works on a PVE Server since I've never been on one but I would assume that everyone does this already for their own accounts.)
Hope that helps..