Buffs. Food/Flasks/Scrolls/Potions/Elixirs are a small, but noticeable, buff to your killing speed and survivability. If you have a max level character that can already supply you with cooking or alchemy, you'll be very well off, because the AH is a cesspool for this. I can't really give you specific foods or potions to look for, since each AH is going to have different supplies. Generally speaking, lower level foods are going to only last 15-20 minutes until you can buy the more powerful versions at 60+. Look for anything that adds versatility or your primary stat, and buy stacks for each level range(1-60/70/80/90/100).
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.
New to Patch 8.1 is a new leveling flask called the Draught of Ten Lands which increases all primary and secondary stats by 20 and experience gained by 10%. This effect lasts for one hour and persists through death, similar to other battle flasks. This item can be purchased using a new Patch 8.1 currency called 7th Legion Service Medal or Honorbound Service Medal which can be earned doing the new incursions and Warfront activities. Simple earn these medals on your main character, purchase the flasks, and send them to your other characters. The cost is 10 of the medals.
As an example, I will take my server values to calculate how many procs you need to make up the price of one Magnetic Mining Pick (40 SS ore, 10 Platinum ore, 15 Expulsoms). Assuming 25g per SS ore, 20g Platinum Ore, and 55x the price of Tidespray linen to craft 1 expulsom in average (got the value from a previous post in this subreddit); 1 Pick is worth 5325g (not counting the elemental flux).
Heart of Azeroth:[5] Our world is direly wounded. Players will be summoned via Magni Bronzebeard to Azeroth herself. She will gift players with a medallion called the [Heart of Azeroth]. Her life essence seeps from the world in the form of  [Azerite]. When players wearing the medallion come in contact with azerite, this sought after resource becomes a source of power for selected pieces of armor, providing players a way to level them up. This feature replaces the artifact system of World of Warcraft: Legion. Currently the plan of development is to limit the upgrade system to 3 pieces of armor. Players will be able to choose the stats they want to better fit their play-style.

Most of the players make the mistake of only listing 30-50 transmog items on the auction house and then hoping to get half of them sold until the next day. Me and a lot of other gold farmers and transmog sellers do have 3.000-10.000 items on the auction house across various realms. It does not mean you have to do it that way – but if you have 1000 items listed, chances are higher you can sell up to 10 per day. With only 100 items listed, you have to be lucky to sell one. Of course, the sells are not guaranteed if you post more items. But i’m sure you have a lot more fun if you open your mailbox and 10-20 items are sold