For starters, if you haven't already, make a new character that does nothing but AH. For example, if you play Alliance, make a Human character and don't bother questing it at all. Just run it to Stormwind and leave it at the AH. THis character takes care of all your buying/selling, all your other characters use the mail system to exchange goods to be sold with this AH character, this saves a ton of time.
Working the AH and/or resource gathering are the easiest IMO not that I'm an expert. I made some of my fastest gold selling glyphs but I'm not an end-game gold farmer, there may be plenty of better ways to make the $$ at higher levels.
I've also made quite a bit selling leather, ore/bars, and herbs. Guess it depends on what kind of $ per hour you would consider worthwhile. The nice thing about AH sitting is you can do very little if you are on a busy server. For example, Put up glyphs and disenchant all the greens and blues people put up for too little then sell the ingredients. I've bought plenty of greens for 3 or 4 gold, DE'd them then sold the materials for 30-50 gold. That's a fast profit, especially for the pain in the ass items that people need a lot of for skill training. Use your AH character to buy the items, mail them to your enchanter, then mail the ingredients back to your AH character.
Tip - if you don't already have it, GET AUCTIONEER ASAP!!! This is an indespensible add-on. While you're at it, get the Postal add-on as well. These two add-ons make life soooooo much nicer for the AH character. Once you start getting to know how to use ALL of the features of Auctioneer you'll be amazed at how easy it is to make gold with the AH.
I have a 30 dwarf that made a couple thousand gold in a few hours worth of buying out one of the Moon Fair (or whatever it's called) vendors every time I walked by my PC then auctioning off all the items.
Another tip - buy out items and resell with a markup on the AH. The trick is to use a dfferent character to mark up 25-35% then use your AH character to mark the rest up 15-20%. This makes your cheaper ones look like a good deal. The more expensive the items, the more money you make.
I'd love to hear from anyone more experienced than me though, I'm not one of those guys that hangs out at gold cap, I just always seem to have enough gold to get everything I need when I need it.
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