This strategy to make gold has yet to be used to its full potential. Some people do take advantage of this but not enough so you can still make a lot of gold using this technique. The only question people have about this is....where do I find the limited supply items at?
First and foremost, what is a limited supply item?

 A limited supply item is an item for sale from a vendor but the vendor only has a very limited supply of items for sale.  These items do respawn...while some take 30 minutes to respawn others take up to 24-48 hrs!

The limited supply items you want to look for to make a lot of gold are usually recipes. I found those to be the most profitable. Typically you can buy a recipe for around 20s and sell it on the auction house for 1-2 gold or more in some cases.

Why on earth would anyone pay 10 gold for something they could buy for 25 silver?

That's easy! People are lazy! They don't want to take the time to go to a specific vendor so they can get the item really cheap. All you really have to do is go to these vendors and buy the items and then place them on the auction house! How Simple!