One of the ways to make massive WoW gold is through the buying and selling of various items. Some of these items are limited in availability, others are only available in one place, and some are both.
For example, the book that teaches you how to take your First Aid skill beyond 150 is available in only a couple of places. For the Alliance it's Deneb Walker in Stromgarde Keep, in the Arathi Highlands.
While most of the vendor items in the World of Warcraft are available in unlimited quantities, such as that book, the limited items might only have one item available. An example is in the Crossroads, in the Barrens. The weapons vendor there sells the Feral Blade, but only one or two at a time.
All of the vendors selling limited items will replenish their stock from time to time, but that time may vary from a few minutes, to hours, to months in the case of a certain vendor in Moonglade who sells a certain dress (this is a vanity item, it has no combat value.) The dress only appears once every couple of months.
Many, if not most, of these limited items are hard to get items, like the First Aid book, will sell for much more on the Auction House than it costs to buy them from the vendor. Sometimes a whole lot more. Do you see some possibilities here?
There are a couple of groups of players who are your target market:. The one group probably doesn't know where to find the item (and doesn't necessarily care,) such as that book, so will buy it off the Auction House. Even though they could save two or four gold by buying it from the vendor.
The next group feels their time is worth more than a few gold, so they buy items off the Auction House instead of spending the time to go wherever to get an item. These people might decide to powerlevel their First Aid skill, so they buy a few large piles of cloth and buy the books off the Auction House. They save hours by not having to travel to buy and/or farm the necessary items.
Then they go to the trainer and raise their skill by 200 points in one sitting. They don't want to waste their time going here and going there to get their book(s) so they blow a few more gold and get it all done now.
The same is true for any other limited item. There's the group that knows it's available from a vendor, but doesn't want to spend the time camping that vendor hoping that the item appears. So they buy off the AH. The other group doesn't know about the vendors, but might not care anyway.
Now your job is this - every time you pass by any vendor in the game you're going to check their inventory for limited items. You'll want to check those vendors several times just in case you missed the item the first time around. Also, you're going to check for items that only appear at that vendor, such as the First Aid book. Then check the Auction House and learn what these items sell for and what the competition might be. You'll be pleasantly surprised in many cases.
Make notes of the profitable items as you find them and build up your own list of these items, then you can make the rounds from time to time and gather up your stock to sell.
The first time you buy something for 40 silver and sell it for 10 gold you'll be convinced. For example, the dress mentioned above has been known to sell for ten times its buy price of 50 gold. Think a 450 gold profit would help your cash flow?
There are a ton of tips and tricks to making massive WoW gold and buying and selling limited items is just one of them. Visit the Azeroth Cookbook for more tips on making gold and other aspects of WoW.