I will list several items that are worth saving, or stockpiling for Cataclysm.
This is a great item to stockpile. Currently, the prices are extremely low, so this is a low risk investment. Snatch up every type of eternal, especially if their prices are extremely low, as come Cataclysm, there will be a demand for those who are leveling through professions.
Look at the past two expansions, prices for essences and primals have always bottomed out at the end of the expansion, and then 6-12 months later, their prices rose, and dramatically. Investing in eternals when they’re so cheap is so low risk that no matter what you do, you’re always going to get all of your initial income back.
What you want are items that are low risk investment, but yield a high return, eternals are exactly this. They’re extremely cheap currently due to the expansion coming to a close. People are dumping their stock because they think that the Wrath of The Lich King materials will become obsolete. The thing they don’t know is that they’re still going to be needed by people, for leveling professions, and making LK Materials.
If you do happen to get burned, you can look at Markco's quick and dirty guide on selling eternals.
Another excellent item to stockpile. Low risk, very, very high returns. You’ll want to keep this as is, this way you have more maneuverability when handling the ore.
Keeping the ore in it’s raw form allows you to prospect the ore if needed, to sell the gems to people who are leveling Jewelcrafting. You can turn them into bars, sell them this way to people who are leveling Blacksmithing. You can also transmute it to titanium bars, which almost always doubles or triples your initial investment, especially if you’re a transmute spec’ed alchemist. Last but not least, you could always vendor Saronite bars and make most of your gold back.
I suggest you grab as much of this ore below vendor price. This way you know you have a “safety net” in case something does go terribly wrong. If you do not stockpile any of this, you’re going to be missing out on huge profits.
Only get these if they are cheap, by cheap meaning under 10g. By getting them under 10g, you’re making sure that you’re losing little to no profit, and that this investment has it’s own safety net. The reason you’ll want these is to shatter them into dust/essences.
Let’s look at the old greater cosmic essence of Burning Crusade, Greater Planar Essence. They’re still used for several enchants, and they are needed to level through the Burning Crusade part of enchanting. I’ve seen them go anywhere from 9-16g each. Arcane Dust, The Burning Crusade equivalent of Infinite Dust, also fetches a good price. I see a stack of this dust go anywhere from 25-40g.
So if the Burning Crusade materials are still selling very strong, let me ask you this, why won’t history repeat itself? The answer is, it will. Wrath of The Lich King enchanting materials are going to rise in price due to people not farming Lich King Heroics anymore, and people no longer doing the old Saronite shuffle.
Another huge plus is that stockpiling Abyss crystals instead of the raw dust or essence, is that it's very space saving, 200 crystals only take up 10 spaces, while 1000 Infinite Dust take up 50 spaces. Space is money, the more space you have, the more things you can stockpile, and yield more gold later on.
On the same topic, stockpiling Dream Shards is not a bad idea, seeing as most of the Wrath of The Lich King enchants require dream shards to get them. Dream Shard Prices have bottomed out, they’re about 1-2g on my server currently, I can see these shards yielding a 5-700% return on your initial investment well into Cataclysm.
Argent Tournament Pets
These pets are currently being dumped at the Auction house for low prices, I’ve snagged roughly 45 under 500g. While there is no safety net on these puppies, I can tell you this, their prices are going to rise, no matter what.
The only way to obtain these pets is to do a bunch of dailies and collect 40 Champion’s Seals, seeing as doing this is located in Northrend, people are not going to come back to do these dailies. They’re going to be busy doing other things, such as leveling, raiding, doing the new Cataclysm dailies, etc. Therefore, the demand for these pets will increase while their supply will remain low, yielding an excellent item to stockpile for cataclysm
This is a medium risk investment, that will yield a very, very nice return. I can easily see them selling for 2.5k+ per pet 6 months into Cataclysm, and even more the further you hold onto them.
Shadowmourne Chest Rewards
There items are going to completely inflate come Cataclysm. There will be so few of them, and not many being put out that the market for these items will rise. Currently you can pick up a full chest for about 200-230k, from what I’ve seen. Now that’s a lot of money, but you can expect a nice return on this investment. I can see these items netting a three to four hundred percent return rates, that initial 200k investment will be worth 7-800k twelve months down the road.
While you are investing a full gold cap on five items, I can almost guarantee you that you will at bare minimum double your gold. There will be so few in supply, even if people do go for the “We are legendary Achievement”, I suspect only top guilds will do this, and even then, if they are a top guild, they should already have a Shadowmourne within their guild.
There is no safety net with this investment, but I can assure you that the prices for these items will rise.
Now, the idea with Stockpiling is that, you buy things for cheap, hold onto them for a couple weeks, or months, while their prices rise. Think of it more so like an investment, such as stocks, or a savings account, you spent X amount of money here, watch it, wait until the market is right, and sell for a profit.
You’re not going to buy these items a couple days before cataclysm and sell them on day 1 and make the profits you’re thinking. If you invest in any of the items above, expect to hold onto them for a bare minimum of one or two months before you see the prices fluctuate. The more time you let pass by, the more gold you’re going to get back on these investments.
Markco wrote a decent guide on every profession, regarding on what to sell before cataclysm, I suggest you check it out and give it a read.