Ah yes, the Auction House- a fickle mistress, to be sure. I have more /played time staring at the AH interface than I do on some of my characters.
RTM's Simple Guide to Auction Housing:
1. Sell every single non-gray item you find on the AH
RTM's Advanced Guide to Auction Housing:
Herein I reveal some AH tricks that I've learned as I've played WoW. In that time, I would guess that I've earned close to 5000g profit just from playing the AH. I like spending it as much as I like earning it, unfortunatly, so I rarely have more than a few hundred laying around. :(
Getting Started
1. Create a level 5 alt and run them to the nearest auction house (I'll explain why level 5 in a bit). Send any items you want to sell to this character instead of running it to the AH yourself. The time you save will FAR outweigh the postage costs in the long run.
2. Download an auction mod. I use
Auctioneer. A mod such as this is optional, but it really helps. Auctioneer has a bunch of extremely handy features- it allows the posting of multiple auctions quickly, keeps track of the prices of everything on the AH, shows you vendor prices of various items, tells you what tradeskills an item can be used in (if applicable), and other nifty things. I originally poo-pooh'd the idea of needing a mod to track auctions, but after awhile it became too much of a hassle to keep track of the vast quantities of items that get posted.
Auctioneer works by scanning all the auctions on the AH at a given time, then compiling that data and comparing it to past data. It can then show you which items are cheaper than in the past. Auctioneer only becomes accurate after a few weeks of regular scanning (once/twice per day).
Posting Auctions
To make money, you need money. To a new player just starting the game, I would suggest taking 2 gathering skills and selling the results on the AH. Some tips for posting your auctions:
1. Search for the item you are selling first to see what it's going for. Undercut the existing auctions by a few silver.
2. If there are 1 or 2 auctions WAY below everyone else, don't undercut them- undercut the price of the majority. Also, buy them out and relist them at the majority price!
3. Prices fluctuate during the course of the week. For example, the trade goods market tends to become saturated on weekends, which drives prices down. Don't be afraid to wait.
4. Set a buyout! WoW players are an impatient bunch. There's nothing worse than finding an item I want with no buyout.
Money-making schemes
Now that you have a bit of venture capital, we can commence with the scheming!
1. Corner a market. Basically, you buy every single item of a given type on the AH and relist it at a higher price. Since all the auctions are yours, people will have no choice but to buy from you. You need to buy out all undercutters and repost them at your price, which requires frequent checking of the AH. Start with low-level small-volume trade goods, which have small AH deposit fees that don't eat into your profits much. I recommend copper bars. Linen cloth can be profitable too, but the sheer volume of linen on the AH at any given time can be prohibitive. It can work if you have a friend willing to check for undercutters when you're offline.
Don't set an outrageously high new price- people won't buy it. Raise it gradually, like the old frog in boiling water trick. Note that cornering a market doesn't really make you any friends and can have long-term effects on the economy. I cornered the market on copper bars last summer for a few weeks and raised the price from ~75s to 1g. It's still around 1g last I checked, but whether that's from my doings or natural market inflation, well.... who knows? :)
2. Enchanting is profitable; specifically, disenchanting. Remember that level 5 alt that you have parked at the AH? Make him/her an enchanter. Disenchanting has no level requirement, so a level 5 character can disenchant a level 60 item. Look for low-priced greens on the AH, buy them, disenchant them, and sell the resulting dusts/essences. Dusts and essences have no deposit fee, so don't worry if they don't sell. For example, Dream dust goes for 60-80s on my server. Level 45-53 greens generally yield 2-5 dream dust. I will bid on any 45-53 green going for 1g or less, which gives me a profit of 60s at least. That's not counting essences which can go for upwards of 3g. Not too shabby! This can work for lower-level greens as well (try Hillman's Shoulders/Gloves)- simply adjust the amount you're willing to pay. I generally stick to armor, since weapons are more expensive.
The auction house can only sustain a few people doing this; the price of greens will inflate to unprofitable levels if there are too many.
3. Speculating is where I've made most of my money. Buy low, sell high. This is where Auctioneer comes in handy- it can tell you if there's an item priced way below its normal price. Buy that item and relist it! A lot of my money initially came from engineering and alchemy recipes. The deposit on recipes is small, and some of the low-level ones (Swiftness Potion, for example) aren't found by farmers like the high level ones. I've bought the Swiftness Potion recipe for ~10s several times, when the going price is around 20g. I used to make a killing on high level recipes like Transmute X to X, but farmers have saturated the market enough with those that it's no longer profitable.
Be careful when speculating on blues- there are a lot of crappy blues out there that get listed for high prices just because they're blue (Moccassins of the White Hare, I'm looking at you). This is where time spent at the AH will help- you'll learn what items are worth what, and recognize when you can make a profit.
4. Resell stuff from vendors. Many recipes are sold by vendors, but people don't know that. Certain recipes are in hard-to-reach or out-of-the-way places, so people are paying for the convenience. As an example, there is a vendor upstairs in the inn in Moonbrook that sells Recipe: Rage Potion for 1s. You can pretty consistenly get 50s+ for that on the AH. Along the same lines, you make a killing from people who use Auctioneer. Buy a lesser-known item from a vendor (like a reagent- Sacred Candle, Maple Seed, etc) and put it on the AH for an outrageous price. By "outrageous" I mean 300g. No one will buy it, of course, but keep posting it for around the same price for a week or two. Then drop the price to a somewhat lesser but still outrageous price, like 100g. The item will now show up for everyone who uses Auctioneer as being 200g less than normal price. Chances are, someone will buy it and relist it at 300g thinking they'll make a quick profit. You can only do this for a little while before people catch on, so make it worth it. :)
Some would question the ethics of the above. I consider it preying on people's stupidity. Auctioneer tells you when an item is sold by a vendor- if people can't read that, frankly it's their own fault and I deserve to have their money. ;) I've only tried this once (Rune of Teleportation wooo!) and it worked with moderate success.
5. Pay attention to patch notes and play the test server. For example, the new 1.10 Tier 0.5 armor sets require a bunch of quests to complete, which in turn require a bunch of trade goods and world drops. If you farm/buy the necessary items before the patch when they're cheap, you can sometimes make good money reselling them once the patch goes live. It's hard to predict what will happen with the economy after a patch though, and I've lost money a few times. For example, in the last patch Blizzard fixed it so that King Gordock in DM couldn't be pulled alone. Farmers were soloing him using teleport hacks and as a result the price of the Ace of Warlords and class trinket books had plummeted. Once the patch went live I bought a bunch of Ace of Warlords & class books, thinking that the price would rise and I would make a profit. Well it didn't, and I didn't. :( If I wait long enough I could probably make my money back but I'm not that patient.
That's all I can think of for now- hopefully it's enough to get you started. :)