A popular way to make gold is by flipping items. What the player does is sees an item posted on the Auction House below its market price, buys it, and relists it for a higher price, hopefully getting a profit from the resale. Good candidates for such purchases are items such as BoE armor items, pets, and mounts that have a higher price tag on them.
To help you scout for good deals, The Undermine Journal has a Great Deals page for each realm, which lists items it has found below market price that you may be able to buy and flip. They are broken down into several categories of items, as follows:
Here you can see an example of the Companion Deals section for a realm. As the help text at the top of the Great Deals page will remind you, you have to be careful when purchasing these items for resale. If you look at the example, you can see that there is a Parrot Cage (Green Wing Macaw) item for a fraction of its market price, but there are several of those items. This particular item...