Potions serve more utilitarian purposes, and it's up to you to decide whether to make use of them. The most common is the Swiftness Potion, which can provide you a handy boost in speed while going after annoying quest objectives indoors. These are generally MASSIVELY overpriced, because people know how useful they are. I opted not to spend the gold since Warriors are already fairly mobile, but slower classes may benefit greatly from these. 

Some quests in my guide are marked as "SKIP" and colored in red.  These quests are simply either too hard to solo or not worth the XP/time and are skipped.  My guide will only list SKIPPED quests if the quest is a direct follow up after completing a quest, not one that you have to click the NPC again to get it.  If you hover over the skipped quests, it will give info on why it is skipped in the guide (unless that info is already listed directly in the guide text).
Prices fluctuated so much because nobody knows what a WoW Token is worth now. Since the WoW Token interface also doesn’t let you know how many tokens are actually for sale at any given time, it’s unclear whether buyers and sellers are rushing into and out of the market at different price points, or whether a single gold-rich buyer using a low point as an opportunity to liquidate can send the price skyrocketing.
First up we have patch 7.3.5, which was implemented several months ago as a major shakeup to how leveling was approached. Instead of going from low to high level zones like the way the game had always operated, Blizzard opened up the map with more of a Guild Wars 2 type system -- you now have more of a choice of where to go with a scaling mechanic. But when combined with 8.0's massive stat overhaul, things got murky.
I found that in most cases, I could pick the zones I liked and skip everything else. It was slower, but I still wasn't forced to do quest zones I hated (and, notably, I skipped the entire Cataclysm expansion's content -- sorry, Thrall.) I did do dungeons, both in group finder and by two-boxing (more on that in a moment as well), but only once for the quests, and mostly to clear what felt like an awfully long slog from 40 to level 80.
Special mention is the Darkmoon Faire Top Hat and WHEE! buff. While not always available due to the DMF being inaccessible most of the month, if you happen to be leveling when the DMF is running, it might be worth your time to go and get these. AFAIK they do stack with heirlooms, although I have not personally tested it. Fair warning: These buffs do NOT stack with each other, and the hats disappear after the DMF goes away later in the month(unless you only log in when the faire is running).

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.
Congratulations on reaching Level 120! There are a myriad of new things to do. The first thing you should do is head to your faction hub ship where you have been doing your mission table quests and footholds to pick up and complete the quests necessary to unlock World Quests. World Quests will provide you an opportunity to grind reputation levels, acquire gear, and gather Azerite. There are tons of new things to do that we will cover in our Level 120 Battle Guide found here: Level 120 Battle Guide