I have not tried this one, since I only got 1 account right now.
But a friend told he makes BIG $ on it
Works with differnt items.

Its actually very simple, but you need 2 accounts, one with an Alliance char and one with a Horde char...

Here goes

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Very good way to get lots of gold is by selling Traveler's backpacks. I do mean ALOT! Usually,: Alliance are two times or even three times more populated than hordes. To start your main have to be on Horde side, because he'll be getting the money. You will also need to have an Alliance character. Farm a little bit money for your Alliance character, when you get about 20g, go to the nearest AH and buy Traveler's bag. Then fly to tanaris, and set buyout very low.

Relog with your main, and buyout those bags. Be sure to do it quick. Ok you got the bags? What next? - Since the Alliance population is much bigger, there are much more bags on AH, and they cost much cheaper. My server on alliance side I can get one bag for 5 gold but on Horde it costs 10 gold. So I bought one bag for 5 gold, sell for 10 gold and I profit 5 gold per bag.

Problem is that some servers (like Whisperwind) have backpacks for about the same amount. Not so fun.

Queues also...