Almost every single patch brings with it new content, and this usually means new gear and more people coming back from time away from the game. For example, in patch 4.1, we had two new 5-mans added. A bunch of new gear was equipped by a bunch of existing and new players, and all this gear has to be gemmed, enchanted, and otherwise improved. All this adds up to an increase in demand for a variety of products.
Enchanting Mats: Time to stock up
One of the first things that feels the supply pinch in a new patch is enchanting mats. New gear gets enchanted, and now that there's a cheap and plentiful supply of Maelstrom Crystals in the new 5-mans, people are starting to enchant their gear with the best-in-slot options. Enchanting mats are such a high-volume market that the prices are quite stable, unlikely to swing greatly from day to day.
Conversely, when there is a swing, there's a lot more money to be made. You can post several thousand dust and have it sell overnight. Part of this is due to the fact that scroll makers act as a mats warehouse; each of them will stock up when the price is good to avoid low margins if the prices go up for a short period.
There are too many ways that enchanting mats enter the market to cover them all, but there are only two that you can cover proactively: buying or making gear for disenchanting. In fact, one of the few features left in Auctioneer that's not (yet) done better by a smaller addon is the ability to parse through the AH and filter down to the items that can be disenchanted for a profit. If you don't use it, stick to crafting for disenchanting. Most professions can make cheap greens and blues, so pick the cheapest one and make as many as you want to disenchant. I'll put together a list of these one day and make a column out of it.
Even if you don't have the desire or ability to actually make mats, you can participate in the patch gold rush by buying them when they're cheap and holding them until the patch. This is a risk because if the price goes down, you're going to lose money. That's not likely to happen after a patch, though. One of the risks of buying for later resale is that the buying pressure you exert initially can increase your price, and the selling pressure you exert later can reduce your price. The more people brokering goods this way, the less the difference in price before and after the patch will be.
This effect, while reducing profit for speculators, actually benefits consumers. When people complain about auctioneers ruining their fun, I point out to things like this. The reason enchanting mats aren't out of stock or unreasonably expensive the day after the patch, even though the number being bought and used has tripled, is that someone decided to stock up and make sure they had some available when this surge in demand hit.
A gem of a way to profit
Jewelcrafters also benefit on patch week. All the new gear coming into the market being enchanted is likely also being gemmed, and that means demand for cut and uncut gems will skyrocket. As with enchanting mats, you can either produce more in anticipation or buy some expecting to resell them.
If you can't cut gems, I'd stick to the obvious high sellers (like cut reds) and avoid the niche cuts, even when they seem dirt cheap. If you can cut gems, just stock up on uncuts, but do your best to get reds more than anything else. At one point after 4.1, the 14 intellect gems I make a point of keeping on the AH at all times were going every 15 minutes, and that's with all my competition posting at the same time. I ran out of uncut reds quickly and wish I had prepared better.
All kinds of stuff
There's a wide variety of items that increase in demand when the patch hits. Leatherworking, blacksmithing, tailoring, and engineering all have item enhancements they can sell. Typically, these share mats with other items, though, and that means that the impact on the buy side will be muted. Still, there will be an increase in demand, so making (or buying) a few extra might be worth it. When I have the time and cheap mats, I've been known to make belt buckles, and they were flying off the shelf for a few days as more and more people got epics out of the new 5-mans.
In addition to demand for item enhancements, some patches will also bring new craftable gear. The 4.2 datamining has shown tons of new PvP and PvE craftables for quite a few professions, and these will likely fly off the AH shelves for the first few weeks as people fill in their missing slots. Not every patch brings this type of new gear, but when they do, you'll have a month or two of notice.
Lastly, when a patch brings class changes, there are often changes to glyphs that could result in a run on one or another glyph. Last patch, we had a new glyph from research, and quite a few mages picked it up. If you're a scribe, follow the patch notes and see if any of them will likely result in a demand surge for a glyph, and be prepared to supply it.
If you're not a scribe, you can only profit off changing glyphs and only if you catch them at a low price. If you're feeling cocky or lucky, you could buy a seed glyph at full price, post it at a much lower price (still above mats cost), and buy all the stock that gets put up just under that.
Maximize your profits with more advice from Gold Capped as well as the author's Call to Auction podcast. Do you have questions about selling, reselling and building your financial empire on the auction house? Basil is taking your questions at firstname.lastname@example.org.