One of my favorite ways to make gold is to look on the Auction House and look for items I can make a profit on. Usually on my realm Arthas working with Ebonsteel Belt Buckles. These things you can easily buy 40 of the mats in weird stacks and then find a Blacksmith and post them up for 12 hours any more and your wasting Gold.
But what pisses me off and other people looking to make gold this way is if there's 1 Belt Buckle and then their undercut by someone selling like 5 or 6 of them! if they waited about half an hour they the Belt Buckle would have sold and your not taking down the market, just today i bought out the supply for 1.5k and reposted 5 for 300g each thinking the market will drop by about 100 to 75 gold nope it drops to about 125 that's lower than when i started!