I'm sure, this is something already covered many times, I'm here just to explain a little bit more the real procedure.
So, let's assume that we want to make some easy real cash but we don't have any good drop to sell directly on the RMAH neither enough gold to make a worthy auction. Well, we can still make enough money just by investing some euros/dollars.
Alright, what you may not know is that Radiant Stars' price is falling on the RMAH while the gold price looks like more stable, in fact, it seems like it's not going to drop below 20 Millions. Here's the easy trick then, we start by buying a gem (Emeralds are more reliable in my own experience) for real money to turn it in a gold auction on the Gold Auction House and in a couple of days, we sell the entire gold made with it for real money.
And now, some math: we spent 19.35€ and earned 20.500.000 Millions, minus 15% fee, we have 17.425.000 Millions left. A million is sold at the fixed price of 2.50€, so the raw profit is going to be 43.50€ and after the taxes which takes away 15% plus another 15% of what we have left we got: 43.50 - 6.52 - 5.54 = 31.44€.
And hey! 31.44 - 19.35 = 12.09€ of true profit.
I also know that people (especially on EU servers) are having hard times while trying to sell gold, thats because you're doing it wrong. Like I said in one of my previous posts, you got to be able to post the gold during the night so, it'll be first in queue in the afternoon of the day after and when it is most likely going to be bought.
Although I'm having growing troubles, I end up by selling always a nice portion of the gold everyday.