This is pretty much common sense however most people I was surprised don't realize this and knowing my luck it's been reposted, could be wrong. Getting a start on gold is easy and can start with as little as 100k, depending on your luck you'll get more profit or less however this isn't buying an item and reselling it for massive profit, although that can be done with this however your profit is going to be measured by this formula: Profit = 105,000 - Price of Item. This formula is subject to change in terms of the constant which is at the moment 105,000 so profits can go up or down.
What you'll do is access the auction house and under "Equipment" put "60 to 60" for minimum and max level and the rarity filter set to "Legendary." Now click the "Time Left" and organize it from least time left to most and scan the pages. This can be done with any time remaining on the auction, I just personally prefer the shorter ones as it's easier to snipe and if someone decides to outbid you, you'll get your money back sooner since people don't really snipe auctions with a day left on them. So you could put in 25k for an item and not even see it back until 23 hours later when the sniping begins. Primarily what you want to look for is the bid to make the most profit, meaning getting the item for the lowest price. It is very important to note this doesn't always work, it isn't 100% flawless and you can sometimes lose profit however that is usually 8k depending on how much Blizzard decides to screw you over by.
Once the auction is completed, send it to your stash and from here you can try to sell it for more than you paid like normal flipping, or you can make a game, and salvage/dench it (WoW Terminology) for a Fiery Brimstone that sells for usually 105k (125k with AH cut factored in) however at times it can dip to 96k.
Sometimes you lose money but usually small amounts, sometimes you gain some decent amounts, I've gained at most 80k as I got a Danetta's Rage luckily for 25k and sold the Brimstone making 105k.
Keep in mind the point at which you begin to lose profit is generally when the bid becomes at 100,000, try to keep it as low as possible, usually when it hits 100,000 I don't even waste my time. If you spent 115k on an item, don't expect to make any money basically. You can somewhat control if you lose profits, the only time you can't is when the other sellers begin undercutting and your item gets sold for 107k as opposed to 120-125k. Also, shift between the types of equipment like Armour, 1-Hand, Off-Hand, etc.
Note this isn't the best way to make money, but it's good if you feel like putting forth no effort. Farming will always make more than this though, a standard act 1 route on gold drops alone usually nets me 60k, that's not counting the two bags of yellows I get. Best of luck. =)