Buying items for Speculation
Buying items for speculation means buying item cheap in hopes of reselling for more. This works, this works well, but this works only if you know your market. Stick to what you know. Make cautious forays into unknown areas to test the waters.
In speculation, knowledge is power. Download, install, and run Auctioneer. Get familiar with it; scan the auction house (an Auctioneer function) frequently (at least once a day) to build up a data pool. Use it for some minor test investments to get comfortable with it. Beware that the data can get very biased, and give false results; you will need to do your own reality check.
Items to speculate on are not merely cheap; there must also be a demand or you will end up with a lot of cheap items sitting in your inventory. Items that are always in demand are:
Quality gear, especially at key levels - see 'twinks' below. Rarely not at least green. Always think, "Who needs this?"
Materials ('mats') - items that get used in professions. This is driven more by use than by source; for example, copper is very easy to mine, but it is widely in demand, and you can often find bargains in copper, bargains you can profit from. Contawise, some very scarce mats may have low demand, may only be used in one mediocre recipe, and may not sell.
Recipes - provide in-game capability to create more kinds of items, and so are always in demand, BUT be careful; if the ingredients are obscure, and the benefits marginal, or the recipe is too common, this is not a good option. Some otherwise very good recipes drop far to often to hold value - Copper Chain Vest comes to mind. This produces an excellent entry-level item, but the recipe is available for low silver at the auction house.
Pets - Reasonably good for speculation, but track demand a bit before you invest. Cute pets seem to do much better than creepy pets.
If you make a mistake in speculating, admit that you made a mistake and move on. Sell the item for what you can to recover as much as you can.
Twink items
Many players who already have higher level characters create alts that they level to a certain point and then stop. Often, these twinks are level 18-19, 28-29, 38-39, etc. for the purpose of going to battlegrounds at the top of their tiers and kicking butt. Since these twinked characters are owned by higher level players with lots of cash, they usually outfit them with the best gear available at their level. Thus, items that require level 17-19, level 27-29, or any other items around this level, with good stats or dps, often sell for much higher prices than they normally would. This is especially true on an older server, and also especially true of blue (rare) items. In general, "good stats" include Cloth "of the Eagle" (for mages, warlocks), Leather "of the Monkey" (for hunters and rogues), and Mail "of the Bear" (for warriors/paladins)as well as weapons with these suffixes that can be used by the right class.
Neutral Auction House
The goblins of the Steamweedle cartel have set up several neutral auction houses about Azeroth. Gadgetzan, Booty Bay, and Everlook all house neutral auction houses. The neutral is useful for making money, as commodities that Alliance players can get easily can be sold at a cheap price to Horde players (or vice versa), and then sold at a higher price at a major city.