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.

This achievement requires you to completely explore both Kul’Tiras and Zandalar continents, uncovering and mapping all available regions. You’ll likely uncover most of the zones without even trying, as questing while leveling then later completing World Quests will take you through most every part of both continents. The achievements will let you know which areas you still need to discover, such as “Savagelands,” but that isn’t all too helpful. Just look for any areas which are still unrevealed on your world map.
There's a good chance you can buy the raw materials for 25% - 75% of the going rate of the finished item, or mine them for free. The fee for an 8 hour auction is only 1 silver, so you can afford to re-auction rods that don't sell the first time. Be careful of making too many rods or other parts of a kind at once, though, as you may get stuck with them for a long time when others produce the same item and set a cheaper price to it.

From “The Burning Crusade” expansion, weapons like Klinge der Zauberei, Die Nachtklinge and recipes for different weapons are really valuable. The best example is Pläne: Teufelsstahlangschwert, worth 500.000+ Gold. These items are World Drops, you can find them in all raids and dungeons. I don’t know any Open World Drops, but that does not mean there are none. Another great transmog source are world bosses and rare spawns on Outland.
While not as powerful as it was before 7.3, this transmog spot is a good area to amass a large number of BOE greens that generally fly around the 35 – 42 area.  It's popularity stemming from the amount of items that drop (and the potential value of those items) rather than the quality of each drop itself. Generally drops from these farms can take a while to sell even when using my TSM posting style.  However if your low on transmog stock this can be an awesome area to hit every now and then.
First of all, the value of transmog is decided by rarity and how they look. Some items like Minenarbeithut der Tiefen or Pendel der Verdammnis (they can drop in Uldaman) are worth millions of gold. They are worth so much, because they have a really small chance to drop. Other items are bought well because they look good. Best example in this case is the Glorious set, its legplates Glorreichen Beinplatte reach a 100.000 Gold value easily.