In World of Warcraft, establishing a monopoly proves much easier than the real world because of less competition and increased laziness from the consumer base, which increases demand through the roof. These factors are easy to exploit with proper preparation. Downloading and installing the following addons from your most trusted website is the first and most important step in the gold making process.
For the purposes of testing this strategy, I started on a new server (High Pop), and the results have been very reasonable after only 3 weeks. I spent 3 hours farming ZF, 3 hours farming RFK, and then 1.5 hours average in WC the following 2 weeks. I currently have about 250 items up for sale, and have made an average of 20k per day over 21 days. Thats 420K gold over 3 weeks, for about 9 hours of work.
Be aware of the seasonality of items. Check what items are used in seasonal achievements, such as Delicious Chocolate Cake or Small Egg during Children's Week. When the Darkmoon Faire is in season, Darkmoon cards and decks (Furies, Elementals, Lunacy, etc.) tend to sell well, but prices also tend to get depressed. When the Faire leaves, prices return to normal, but sales volume decreases. The same is true of things like Snowman kits, Red Holiday wear, etc. Holding onto that Snowman kit for a few months, and then listing it in July, can net you a significant profit.
The vanilla WoW Horde leveling guide you see on this site was originally made back in 2006 by Joana (AKA Mancow, or FuriousPaul).  The guides have been tweaked many times over the years to make things "faster".  The leveling guides were made because of all the messages I got from people asking me how I was able to level up so quickly on new realms.  I was first to level 60 on 7 different realms (3 with Mancow and 4 with Joana), even winning Blizzard's "First to Level 50" contest they held back in 2006.
Battle for Azeroth (110-120) Leveling Tips and Consumables How to level from 110 to 120 as quickly as possible in WoW Battle for Azeroth, including recommended zone order, best consumables, general leveling tips, and best addons. Leveling from 1-60: Questing and Dungeons Recommended leveling paths for new players between 1 and 60 in World of Warcraft. Includes tips for questing zones and leveling dungeons. Heirloom Collections Guide A guide to the Heirloom Collections Tab. A complete list of all heirlooms, how much they cost, which vendors sell them, what currency you need, and more!

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.