I make most of my money by farming the Auction House.
I used to have the best system I would get such easy money by buying cheap socketed level 16-35 items, putting a red gem in them to put them on the front page of DPS for that level, then jacking up the price by a factor of sometimes 100 or so. They would sell instantly. Unforunately the removal of gems as a DPS factor ruined my scheme.
I did have some amazing flips though. I'd get like a 30.7 dps socketed 1 handed crossbow for 2500g put a gem in it to get it up to like 38 or something, which would make it #1 or #2 when you search by DPS (which is what causals do, its the easiest), and sell it for like 200k. They sold so quickly to. I miss that.
Now I've been farming Inna's Favors and Zunimassa's trails. But the market has collapsed for those (Probably I'm partly to blame). I buy the cheapest ones that just go up for like 1.2m-1.5m, then sell them for 1.8m, make a quick 200k-300k, and just do it over and over again. You just have to buy basically the entire first page so that the minimum buyout left is over 2m, then you are basically undercutting everything.
Yeah.. I think about this too much.
EDIT:
Here's a couple Trail's and Favor's from a few days ago. You can see that even after blizz tax I get a solid profit of around 200k for each sale (for a 1-2m initial investment), and the turnaround is pretty fast. Unfortunately like I said the market is dwindling a lot.