This one is probably already known here but I haven't seen any other posts about it so I figured I'd post it anyway.
First, create an alt on the same realm as your main.
Take your main and go check the auction house for rare or epic items. Try to find an item that only has one listed. Twink items work great for this. This will not work if you choose an item that has multiples on the AH. For this example, we will use
Feet of the Lynx for 60g. Make your purchase of the
Feet of the Lynx for 60g. Relist the item in the AH for 100g. After you have relisted the item, it's time to find a sucker. Log into the game with the alt that you just created. Type into the trade channel "WTB
Feet of the Lynx. Willing to pay up to 120g"....or something like that. Most players that like to make gold on the AH will check to see if the
Feet of the Lynx are on there. They will soon realize that the item that you are willing to pay 120g for is on the AH for 100g, and they can make an easy 20g by purchasing the item and re-selling it to you. After they send you a tell and offer to sell you the item, agree to meet with them (IF bridge....SW Bank, etc.). Once you meet them, say something like "brb in a sec...I have to send over some gold from my main so I can buy it". Log out with your alt, and log on with your main. Check the mailbox to confirm that he just purchased your item. No need to log back on with your alt at this point. You just made an easy 40g!