Quality gear. These can be greens, rares, whatever. However, always think, "Who needs this?" Items that have stats like Stamina are typically useful to all players. Items with stats like Spirit are only useful to a subset. Items that combine two highly sought-after stats, like Stamina and Intelligence (which all casters need) will sell for more than items that combine two stats like Agility and Spirit (which practically no class needs).
The bones of Mythic+ are fairly simple, but it’s that simplicity that makes it Battle for Azeroth’s most rewarding mode. It’s fun, generally only takes about 40 minutes per dungeon and allows small groups of players to push themselves exactly as hard as they’d like. Battle for Azeroth’s dungeons are extremely well-designed and consistently entertaining, even after doing some of them 20-plus times, which makes the challenge of Mythic+ a great way to keep people playing.
The expansion is set immediately after the events of Legion. Two continents have been added within the Great Sea between the Eastern Kingdoms and Kalimdor: Kul Tiras, one of the major human kingdoms, and Zandalar, the homeland of Azeroth's trolls. While the Alliance and Horde will initially travel to one continent respectively, both continents will be available to both factions at level 120.
A "stat squish" and "item squish" was implemented to lower the numbers used in the game, e.g. a legendary item previously with a level of 1000 reduced to 265. Unique class-specific buffs are added back, i.e. mages' Arcane Brilliance and priests' Mark of Fortitude. Titanforging—a random event that raises the initial item level of an item gained via drops or rewards—is still in the game; however, if the item is one of those affected by the Heart of Azeroth, then that item cannot be titanforged.
Tiragarde Sound and Stormsong Valley are both fairly similar zones, but Stormsong is a little longer, meaning that you might not even have to finish it to hit 120. Tiragarde also comes with a bit of a warning. There are plenty of side quests to do in the zone after you finish the story quests. Don’t do them. It’s not worth the time or the experience you’ll get.
With a bit of upfront time investment, and luck, you can kickstart your transmog business. What I recommend is to spend 6-8 hours, or however many you can stomach, farming ZF, WC, and RFK over the course of a week. This should net you a few hundred items of value to list on the AH. Over the coming weeks and months, spend at least one hour per week, revisiting one of these dungeons, or branch out into other classic dungeons.
Elixirs serve the same purpose as Flasks, but the buff expires if you die. You'll only use these between 1-70, generally speaking, until you can start buying Flasks which persist through death. Try not to die, and carry extras just in case. If you can't find an Elixir with the stat you want, look under scrolls, since they count as battle or guardian buffs as well.
A dungeon should take about half an hour to 45 minutes, that is reasonable. What did we do to Maw of Souls because it was by far the fastest dungeon to spam run for AP? Nerfed its yield. Same happened to dungeons. Except instead of nerfing the XP, they adjusted the clear speed to justify the experience a bit more. Is it perfect? Nah, a lot of dungeons aren't even that long with updates. At most a clear takes 5 mins more.
I recommend not counting on transmog sells for your monthly gametime. Its a great market, but also pretty random. You have to have the right items posted at the right moment if a player needs a certain item. If this is not the case, of course you will not sell anything to him. There are people making millions of golds a month with transmog. But they are very professional at it and it really takes a lot of time.