It's best to put stacks of five and one on the AH. People will buy singles if they have some already, and the stacks of five will sell briskly to people who don't have any in their inventory. Remember that you don't always have to be the cheapest auction to make a sale. You can sell a high quantity by being cheaper than the largest volume posted; for example, if the lowest price is 2g but there are only about 30 eggs at that price, you could post yours for 4g and they'd still sell. Demand for these is very elastic because if the price gets too high, people will just go and farm it themselves. Decide whether you'd rather move a smaller volume for a higher price or a large volume for a smaller price.
Monopolists, beware: Buying out everyone below your chosen fixed price is a very bad plan here unless you choose a low enough price that people are still willing to buy. If you force the price up to, for example, 10g an egg, you'll stop most people from buying them and find more and more eggs posted for 5g, as people realize that you're buying everything below 10g. The higher you try to fix a price, the more risk you have of ending up with a ton of stock that cost you a lot more to acquire than your competition paid.
Ice Cold Milk
Bear with me on this one. Ice Cold Milk is purchasable at the inn for a couple of copper, but some people are too lazy to look up where it comes from and will simply buy it at the AH -- enough of them that the average price per milk on the AH is up to 90s each at the time this was written.
This market is a little strange. Like vendor recipes and sometimes Dust of Disappearance, when people are sometimes willing to pay more at the AH than they would at a vendor, the market dynamic is very different than what we're used to in the world of crafted or farmed goods. There's no limit to supply, so in theory, anyone with the time and bag space to do this could post thousands of milk on the AH for a couple of percent above cost and it would never be worth selling.
That said, this takes time and bag space and doesn't generate a reward other than knowing that people unwilling to walk 30 seconds to get to an inn can now get the same price as people willing to walk 30 seconds to get to an inn. Long story short, the supply is generally limited by the reward inherent from selling the milk, so when it's higher priced, there's more of an incentive to buy and list, but when it's low, it's not worth the trouble.
Everyone working on achievements is going to try to get some Winter Clothes crafted. These are made with a ton of Runecloth and some Woolen Cloth. The boots are crafted from Wool and Rugged Leather. If you have any of these mats in your bank, you'll find that they sell for many times more now than they normally do. If you're a low-level character looking for some easy money, farming Wool and Runecloth can be done in lower-level instances, often soloable, depending on your level. Wool, in particular, comes from The Stockade, a low-level instance with its entrance in Stormwind. Horde players who can fly should be able to fly straight in without much trouble if they can avoid the flying guards.
Speaking of holiday-spirit city infiltration, there's a small but affluent market of people who like wearing things you can't get without some pain. The vendors who sell these recipes sell green clothes in Horde cities and red clothes in Alliance cities. They're not picky about who they sell to, though. If you can fly in fast enough and get to them, you can pick up the recipe from your opposite factions' vendors. If it helps, here's a macro that you can bind to a button or that will buy the recipe without forcing you to find it on the vendor: "/run BuyMerchantItem(10,1)"
Changes since last year
Last year, we were able to turn Deeprock Salt into Preserved Holly, which, unlike the fresh stuff, would allow people to get the Fa-la-la-la-Ogri'la achievement. The salt went for as much as 35g while the daily was up, so a lot of people had been stockpiling it for this year's event. Unfortunately, Blizzard seems to have deprecated this particular quest, and I can't seem to find any other way to get Preserved Holly.
Maximize your profits with more advice from Gold Capped. Do you have questions about selling, reselling, and building your financial empire on the auction house? Fox and Basil are taking your questions at firstname.lastname@example.org and email@example.com.