Usually when a wow expansion is nearing, people tend to do a bit of bag cleaning. And by cleaning I mean valuable trade goods are being sold and undercut like crazy, at least on the more popular realms. A great many people are also saying goodbye to their garrisons and spending their last resources on trade goods. Usually the most valuable, so except the prices to get affected as well!

Anyway, however you choose to get to the addon's menu, choose guide list and then pick the zone you want to quest in. There are some zones that will autoselect based on quests in your log, but not all guides or all quests are set up that way. (If you don't want to switch to a guide that auto-pops up choose "not now" or "never") As you can see in the screenshot, there are various categories of guides. Most max level content (such as Argus) is found in Achievements rather than leveling.
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.

 Zandalari troll (8.1.5) - "The Zandalari are a proud people whose origins date back to the early days of Azeroth's history. Their fierce warriors ride dinosaurs into battle, and they possess one of the most formidable naval fleets in the known world. But enemy attacks on their borders—as well as unrest within the king's council—have threatened the kingdom's foundations. By returning stability to Zandalar, the heroes of the Horde can gain a powerful new ally.."

XP Potions: The Elixir of Rapid Mind and Elixir of Ancient Knowledge no longer drop. They can be bought on the AH still but are way to expensive for it to be worth it. The Excess Potion of Accelerated learning is available from your garrison quartermaster but only works for level 91 to 99. As of Patch 8.1 you can also buy the 10% XP Potion Draught of Ten Lands.
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.