When it comes to the auction house, it is easy to lose a lot of gold if you don't know what you're doing. So here are a few things to get you started in the right direction:
#1: Predict The Future
It is possible to predict what items are going to be in low supply and high demand in future. If the notes claim that a new instance is going to open, for example, and it looks like it might need a certain item, then stock up on that item. You'll then be able to cash in when their value goes up.
#2: Be Patient When You Buy Items
If there is an item you want to buy, be patient about it - and try not to buy above the market price unless it's really important. Simply set the maximum price you're willing to pay, and then wait for the offer to come up.
#3: Be Patient Selling
When you're selling an item, don't just take the first offer you get. If you keep your item on the auction house longer, you'll get more exposure, and probably more players who are willing to pay more for your item. If you get a good offer you can still hold for a little while you check your other options.
#4: Act Casual
Here's something that has worked quite well for me. Basically, if you get a low offer for your item, act like you don't care and simply ignore the potential buyer. Often, the potential buyer will come back a few minutes later and offer more for your item.
The reverse here is also true: if you make a low offer for buying an item and act like you don't care, many sellers will give it to you. Just don't come across as desperate whatever you do. My experience is that many players are not good businessmen, so tactics like this can often work.
#5: Use The Auctioneer Mod
When it comes to making gold off the auction house, the auctioneer mod is crucial. This will scan the market price of items and allow you to automatically find bargains and then resell the items at a higher price. Just Google "auctioneer" and find a place to download it for free.
