Some players even turn this into their profession by systematically "plundering" vendors in the game world and then selling the items on the auction house at a significant markup. The reason why this works (even for items which are on unlimited supply at vendors) is, that many players don't want to spend time traveling to specific vendors to get hold of a recipe or skill book. They would rather pay a slightly higher price at their local auction house. In some sense, they use the auction house as a "super market" or "convenience store". So it is completely reasonable and legitimate to be the supplier for this convenience store and make money out of it.

Once your auctioning operation reaches a certain size a second account becomes an invaluable tool. It will allow you to run auctioning scans, shopping scans and collect mailboxes while playing your main. I strongly suggest buying a second monitor as well. This will help you really unlock the benefit of a second account. For a detailed guide on why and how to set it up check out my post on the subject.
As a hero of the mighty Alliance, journey to the seafaring kingdom of Kul Tiras, home of Jaina Proudmoore. Untangle a web of betrayal and dark magic as you encounter power-hungry pirates, witches wielding death magic, mystical sea priests, and more. Explore the stony peaks of Tiragarde Sound, trek across Drustvar's high plains and red forests, and navigate the intricate inland canals of Stormsong Valley as you convince this fractured kingdom to join your cause.
The most simple thing is TradeSkillMaster. This AddOn shows you quite an amount of information in the items tooltip. In my opinion, DBRegionMarketAvg the best source. It tells me the price, an item was posted on all EU realms in the last 14 days. Why not DBMarket? DBMarket only shows you the items value from your server in the last 14 days. This value often gets manipulated. Some people start to post an item with a 500g value for 500.000 Gold on the auction house for several weaks. As a consequence, DBMarket is rising towards 500.000 Gold.  If you use DBMarket with 100% and post your item for 500.000 Gold, nobody will buy it.