It seems that the biggest consensus is that auction house flipping is the best way to make money in Diablo 3. For myself, I never flip on the auction house simply because I’m not entirely interested in making gold in this manner. However, I have to point this method out since it seems to be wildly popular.
Essentially, auction house flipping is taking an undervalued item (or bunch of items), purchasing them out then re-posting them for a higher price. To do this you require quite a bit of gold as starting capital to make the right investments. Second, you really need to know the market. In this case, you need to know what are hot items and the stats that players require for an item to be valuable.
One of the chief problems in the game, imo, is that many items have ridiculous price listings. You’ll even see worthless items being charged an insane amount. As a result, the economy on a whole can be rough when it comes to judging how much an item truly is worth.
Now, I’ve seen a video which demonstrates how you can get closer to selling an item to what it’s worth. You essentially need to search for items with similar stats or lower. I think that this methodology mostly works on higher level gear, legendaries and set items. I find attempting to sell rares to be quite difficult for higher levels. But the main thing is attempting to corner a market.
How can you do that? First, you need to know what the hot spec is and what a build requires to make it work. For instance, at the moment Whirlwind Barbarians are known to be the most popular for farming. So you would look for gear such as fast off hand weapons with life on hit, barbarian belts, high critical hit chances, etc. on top of all resist gear. Next buy out items that you can afford but put them back up for slightly higher prices to make your profit.
The disadvantage to this methodology is that you’ll end up being forced to spend more time on the auction house interface, sniping and waiting for your auctions to work. However, it does has a better chance for reward in the short term since you just need a few items to sell higher than what you paid to really make your money.