Generally speaking there are plenty of ways to earn cash, you just have to find the one that fits you best. I will try to show you a couple of ways that worked for me and some advice that I learnt the hard way.

Grinding:

The most obvoius way is all about killing mobs and hope for good drops. Personally I prefer to grind mobs that serve multiple purposes – like giving reputation, dropping special items in addition to normal drops, etc. For example the mobs on the Blades Edge Plateau's Etherum Prison Keys (if you completed the quest), Netherweave Cloth, Greens, Blues, Epics, depleted Items, Motes of Mana, Apexis Crystals, … there are plenty of mobs and fast to kill.

• Scarlet Archmage; Runecloth (= Dream Dust, Eternal Essences), Formula: Enchant Weapon – Crusader
• Spectral Researcher; Runecloth (= Dream Dust, Eternal Essences), Formula: Enchant Weapon – Lifestealing
• Talonsworn Forest-Rager; Mote of Life and lots of different Herbs
• Animals in Nagrand or Terokkar for Meat and Leather, Clefthoof Meat, Talbuk Meat, Warped Flesh, Blackened Basilisk

Of course there is also grinding by gathering. Just check AH, which resources are low Mountain Silversage, Terocone, Netherbloom, Nightmare Vine, Adamantite Ore usually sell very high on my realm. Much higher than the Potions and Elixirs you can make of it.

The best farming time seems to be late night and early morning. If someone else is on your route you can easily swap the direction of your route, change the zone or resource you are gathering.

The more professions you have access to via alternate characters or friends the more you can make of the drops you find. For example I have swift flying on a gathering char to get to minerals and herbs as fast as I can. The characters that crawl through instances have enchanting. The Jewelcrafter (prospecting), Tailor, Blacksmith, Engineer, Alchemist do not need to be high level characters since you need only level 50 to get 300+ max skill in these profession and it is enough to max the profit you can do with these professions.

With the exemption of herbs raw materials usually sell for less than the products you can craft from them. Just look at a stack of Runecloth. It sells on this Realm for 2.5g/20, but if you create Runecloth Belts and disenchant them you get 2-5 Dream Dust (3-7g), 1-2 (6.75-13.50) Lesser Eternal Essence or 1 Small Brilliant Shard (9.95g).

Awesome profit comes for example from:

• White Bandit Mask, cheap source for Vision Dust, Nether Essences, Small Radiant Shards
• Runecloth Belt, Dream Dust, Eternal Essences
• Wicked Leather Headband, the only source without special drops for Illusion Dust, Greater Eternal Essences, Large Brilliant Shards
• The Sharp Cookie or Avenging Blades, cheap merchant buyable source for Arcane Dust, Greater Planar Essence

Prospecting: 

Prospecting Ores is more gambling than a secure way to gain cash. Personally I like the excitement that comes with it and you can have great luck. Here is a nice list of prospecting percentages and which ores to expect.

Auction House:

So how do we gain the most out of the items we grinded? I suggest that you have a basic cache of money that will finance your daily expenses. This way you do not rely on the cash you get from the auctions and do not have to undercut the lowest price and you will gain a steady income after a while.

Even if you are not the cheapest seller, you will still sell plenty of auctions, after the cheaper ones are out, or when you manage to list your auctioned items at the first pages. Even after 2.3 it is still possible to gain money this way, since not all players sort the auctions after prize. There are stilll a few tricks. It seems that the auctions are sorted by the min bid price and not the buy out. If you sell your item for 1.99 bid and 2.50 buy out and someone else for 2.00 and 2.25 buy out your auctions will be listed as cheaper and most players use the buyout function anyhow.

On my Realm you can get Arcane Dust for example as low as 1g per unit and I still manage to sell mine for 2.75 bid 2.85 buy out.  do that by listing masses of auctions. Since I can't compete with single sales or with whole stacks I specialized on stacks of 4 Arcane Dusts. Most people do not need 20 Arcane Dusts so they rather buy 4 for 10g than 20 for 20g. You will be listed after the single stacks but in front of the big cheap stacks.

From my experience Sundays, Mondays and Wednesdays seem to be the best days and 17:00-20:00 and 23:00-0:30 the best sale times. These are the time frames where your items should be at the first pageof the auction house. If you use this smart you can sell the same items as other sellers with a significantly higher price. I also noticed that you should always include a buyout with a little higher price, this will encourage last minute buy out to "save" some money.

Dominating the Market:

Basically you buy out all products below a certain price and resale them for more. If you decide to try that you have to choose the procduct careful. There should not be too many auctions of the item, it should not be too easy to get and the deposit price should be low. If you try to dominate a large market you will need a lot of money as reserve.

It also helps to have a few friends at your side while trying this. Many players look at the auction before they place their own items and many different characters with the same high prices will raise the overall prices. 

For example I tried to dominate the market with Arcane Dusts as it seemed to be a stable market and even if I'd have failed I would not have lost a lot of gold, due to stable prices. I had 9500g when I started to buy all Arcane Dusts that were cheaper than 2.75g each and put it mine up for for 2.75/2.85. People started to dump all their reserves on ah only slightly cheaper than mine. I spent around 8k gold before the first of my auctions started to sell, this nearly killed me. The income was much lower than the amount of dusts that were dumped on the market. But with the Tipps I gave above I was able to make quite some profits there.

Items:

Which Item sare good for sale? It is really hard to generalize this for all realms, but from my experience you gain a fortune with mats that others need to level their professions. For Enchanting for Example there is no way around Vision Dust, Dream Dust, Illusion Dust, Eternal Essence. Usually You do not sell a lot of these, especially if You sell them expensive. But players that level up their professions need those in large masses and are willing to pay a steep price.

Addons: 

Auctioneer is an easy to use tool to put up large masses of auctions. It saves the prices you are using for your auctions, stack sizes and shows an overview of competition ont he items you are trying to sell.

Auctioneer includes Btm-scan, a tool that shows auctions which promise a certain amount of profit. The longer you use the tool, the better results it will provide.

Large amount of auctions mean a large amount of mails. A great help dealing with masses of mails is Bulkmail2.

Possessions is an addon, that lists items in inventory, bank and mail for all characters on the account.

I will be updating this post regularly, feel free to ask questions.

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