Probably the best way to make gold in Diablo 3 is to sell unidentified legendaries and set items. These are the orangish brown items and green items which sometimes drop. Some of these items can roll very good stats and are highly sought after when they do roll well, whereas others are typically considered to be pretty subpar and not many people really want them.
Some items to hold onto or even buy from others, if they are cheap enough, would include:
-Manticore (legendary 2 handed crossbow)
-Ceremonial Knife (set item - will be Manajuma's Carving Knife)
-Gory Fetch (set item that looks like a chicken)
-Skorn (legendary 2 handed axe)
-The Witching hour (legendary belt)
-Echoing Fury (legendary 1 handed mace)
-Chantodo's Will (set item - wand)
-Calamity (legendary 1 handed crossbow)
-Won Khim Lau (legendary 1 handed fist weapon)
-The Butcher's Sickle (legendary 1 handed axe)
-Mempo of Twilight (legendary head armor)
-Chantodo's Force (set item - wizard source, off hand item)
-Ice Climbers (legendary boots)
-Natalya's Reflection (set item - ring)
Natalya's Bloody Footprints (set item - boots)
This is not an exhaustive list and pricing isn't included here. However there are some forum threads, YouTube videos, blog posts, and so forth where you can get some of this information. If you play on European servers you might want to check out this thread: http://eu.battle.net/d3/en/forum/topic/7195039570 which more or less just links to this spreadsheet on Google Doc's EU UNID prices here.
For those of you who play on America servers you might find this thread for America region UNID prices handy.
This will of course have a bit of a learning curve and you will end up having to put a bit of time into the game to learn the markets, keep up to date with the markets, find legitimate buyers, and so on.
Tip: DO NOT click & drag items or gold into trade windows in game. There are people who will close the trade window on purpose and you will end up dropping whatever is on your mouse that you are dragging. They will then pick up whatever you dropped, disconnect from the game, and leave you high and dry. This is just one of several scams some people will try to pull. If you are trying to trade with someone and the trade window keeps closing, just drop the deadbeat and try the next trader - because this one is trying to rip you off.
Also keep a very close eye on the amount of gold, as some people will try to drop a zero or two at the last moment before the trade goes through. At this time there is no word from Blizzard regarding fixing either of these issues.
Once you start to learn the market value for various items this will make working the auction house more feasible. However you should not spend more than an hour a day at most in the auction house, unless you have a huge pile of gold, know the markets, and are making a minimum of around 50 million gold profit per flip. Otherwise you are really pretty much just wasting your time. You could go clean your neighbors gutters for $10.00 and just buy around 400,000,000 gold and call it a day.
Another handy method once you have gold to work with, maybe have some buyers, and have a good idea of the going prices for various items is to simply try to buy UNID legendary and set items in trade channels or even from people in various forums online. For instance if you can buy an unidentified (UNID) Witching Hour for 500K, or maybe even 2 million or so, and then sell that item to an UNID buyer for 5 million, that is a great profit - and you won't even have to share your profits with Blizzard!
This is probably the best way to make gold in Diablo3 legitimately and still actually play the game.