Cross faction

If you have the time and a allie toon you can start doing cross faction trade to start you need to transfer about 1k to alliance and scan the auction houser several times for about a week to have good data. Then you go onto the search tab appraiser in auctioneer, and set up the same way you set up resale 64% and 1g minimum. Then buy the things that comes up and send it to a toon you have in booty bay or another neutral ah and buy it on the alliance side and sell it. The neutral auction house takes 15% of the amount you sell the auction for so the best is to post if for a couple of copper but then it can be snatched up by someone else thats camping the neutral auction house so you need to be quick. Of course you need two accounts or a good friend to help you with flipping sides of the items.

So what to flip sides with? I would say pets,trade goods and things like promodial saronite. Basically everything that's harder to get at one side but is not faction specific.

From 20k levling forums:

Personally, for cross faction I just look at common items and then take advantage of market discrepancies. Main ones I look at are: Titansteel, Titanium ore/bars, Primordial Saronite, Cloth, Crafting and cooking mats.

On my server I will see primoridals at 200-500 gold difference, titanium around 20-100 gold, titansteel around 20-30 gold, cloth 5-15 gold per stack (rune cloth, wool, linen are the ones I check) and then crafting and food... Fish mats can be 30 gold difference, crafting mats can range a bit too. I just moved my dk from skinning/mining to blacksmith/jewelcrafting and much of my mats I would buy more than I needed on alliance and sell the excess on horde and basically pay for all of my mats that way. I had 8k gold on my dk when I started and I finished with 10k after I got done leveling profs. Primordials are cheaper on horde but the rest are cheaper on alliance.