Back before Cataclysm, everybody was talking about what to stock up on, what would sell well and make us all rich. Netherweave and Frostweave bags were two items that came up frequently. Everybody knew they were "can't-lose" investments, because new characters all need bags, right? And sure enough, as soon as Cataclysm hit, bags started flying off the Auction House, and prices started rising. But that wasn't the best time to sell bags. Now is the best time to sell, and if you have (or make) a good supply of bags starting this weekend, you're going to make a killing.
Why is that? There are two reasons. The first is that a lot of people haven't really started playing Cataclysm. Maybe they've bought it, installed it, and played around a little bit, but server queues can be discouraging if you get home from school or work to find an hour wait in order to log on. A lot of people will have been waiting for winter break to really get into it, and that's starting this weekend. A lengthy vacation means a lot more people playing, which means a lot more people needing to buy things.
The second reason, and the one that's more interesting to me, is competition. Everybody knew that bags were going to sell great when Cataclysm launched, so everybody posted their bags for sale....and now everybody's sold out of bags. A lot of people don't want to spend an hour making more bags to sell when they could be out enjoying new content (have you SEEN the craft time on Netherweave bags?). As soon as Cataclysm released there were hundreds or even thousands of bags being posted, and now they've all sold. As of this morning, my server's auction house had 0 (yes, ZERO) Netherweave or Frostweave bags posted for sale.
There was a huge increase in demand for bags immediately after Cataclysm launched, but there was also a lot of supply available. We saw some price increases in those first couple of days, but only by a few gold per bag. Now we still have a high level of demand, but the level of supply has dropped by quite a bit. High demand plus low supply equals high prices. I've seen Netherweave bags go from 10 gold pre-Cata to 15 gold in the week after release to 30-40 gold now, while Frostweave bags have gone from around 70-80 gold to over 200 gold.
If you're not in the bag market, get into it. If you sold off your Cataclysm stockpile of bags already, check your server's AH, because it may be worth it to get back in. And also, the bigger lesson from what's happened with bags: predicting market trends is great, but predicting your competition and how they'll react to market trends is even better.