[Dazarath]

Here's what I've learned in my short time trading on the AH. Just a warning, I've only been doing this for the last week or two, but I've probably spent as much time trading as I did grinding. Also, I'm including trading in the trade channel along with trading on the AH. It's all the same to me.

First of all, get the add-on Auctioneer. Learn how to use it. Also, you can use Allakhazam.com and look up median AH prices on that site. Between those two tools, you should be able to get a good feel for what an item is worth. Auctioneer has a built in tool to scan for auctions below the market value. Of course, you don't want to just scan and then buy every item on the list. You need to eyeball the options and make good picks.

I try to pay attention to the trade volume of different items. This will determine how easily you are able to unload items after you buy them. It's just not worth it if you pick up an item for 1g but it takes you a week of "WTS xyz item" to unload it for 5g. I will categorize things below.

Green weapons/armor (tradability: low)
I personally think these are terrible. (These can be good to disenchant if you can get them low, but I'm just talking about straight trading here.) If you post them on the AH, it's so unlikely that someone is going to be looking for your specific item, so that basically means you have to sell it in the trade channel. I personally think this is a huge pain and I don't deal with these. Tried a few times, got stuck with [censored] items.

Blue weapons/armor (tradability: medium)
These are a little better, but you need to have some idea of which ones are good/bad. As an example, casters use cloth, so stats like intellect, and to a lesser degree spirit and stamina, are useful. I haven't played any of the fighting classes so I don't have that great idea of what amount of which stats are good. In general, I try to avoid these unless I can get an amazing deal, like < 25% market price, or if I know it's a high demand blue.

Purple weapons/armor (tradability: high)
These are the easiest to trade out of these 3. People want epics, and it's so much more likely that someone will do a scan of the AH for new purple gear, than blue, and moreso for blue than green.

Green recipes/books (tradability: low)
Don't bother.

Blue recipes/books (tradability: medium)
Some of the recipes are really high in demand (ie. fiery enchant, mongoose). I find that recipes < blue are not worthwhile, and even a lot of the blues are crap. The prices for the books (to teach spells) seem to fluctuate a lot and I've found that sometimes I can pick up something like Book: Gift of the Wild for 1g. But then it ends up taking me awhile to unload it, because 24h in the AH isn't always long enough, even if I'm listing it at 75% market price.

Purple recipes (tradability: low)
I've had a couple experiences with buying epic recipes for 500g that were supposedly worth 600-700g. My theory is that these are pretty rare, so if someone wants what you're selling, it's not going to matter to a rich 70 if it's 600g or 650g. You might not want to jump into these right away, as you'll find you can be stuck with them sometime. And it's hard to get a good estimate on the price. If you make one bad buy on a recipe that Allakhazam lists as 600g but it turns out you're server values it at 400g, you could be out a good amount of gold in the process, wiping all of your small 5-10g trades.

Goods used in crafting (tradability: high)
These include ore, cloth, dusts, essences, etc. In general, I'm not searching these for deals too much. It should be obvious that items with higher trade volume also tend to be more efficiently priced. I think if you get into this stuff, you'll find yourself nickel and diming for 1-2g at a time.

Cards (tradability: varies)
If you're going to buy/sell the cards (you know, the furies, blessings, warlords, etc cards), make sure you know which ones are good. I picked up the 2/3/4 of warlords (3 of each) at ~1g each, and I found myself having trouble unloading them for even a low price. On the other hand, the blessings cards are really in demand. I think these can be worthwhile if you're smart about it.

There are other items too, but I'm getting too lazy to list them all out. Basically, I look at the tradability of items. The higher it is, the lower profit margin I need to make it worth my time. If I see someone selling a stack of arcane dust for 12g, and I know I can immediately turn around and sell it on AH for 15g and it will be bought, then sure, I'll do it. On the other hand, if I can buy a lower traded item for 12g and eventually sell it for 15g after WTSing in the trade channel everyday for a week, forget it. You should look at your profit margins in nominal values rather than relative values. Making 50g on 200g purchase is obviously better than making 2g on a 2s purchase.

Items that you know people will search for, like dusts, ores, etc. Basically the high volume items. It's better to list these in AH. Much simpler on your part and you can dedicate your time elsewhere. Items with low trade volume should be sold in the trade channel.

Also, understand the idea of undercutting. Sorry, it's just a stupid joke I'll make because the kids who play WoW are sometimes way too young and stupid to understand the idea of basic supply and demand. I see people do this all the time: "Undercutting prices blah blah blah" and then I go take a look and I see 10 singles of item xyz for 10g and that guy's 5x stack for 60g. I was initially skeptical that an MMO which has been around for awhile would have such an inefficient market, but I was proven wrong very quickly after spending some time in the AH. In general, if I buy an item for 10g and I can sell it immediately for 17g or spend some time and squeeze 20g out of it, I will take the first option. You only have so much time/chat resources to spread among trading your different items, that instead of focusing on an extra 3g for this one, you could be focusing on acquiring a new item for 7g.

I'll post more if I think of anything, but I'm hoping that someone else will contribute their little tips/tricks that they've picked up during their playtime.