In real life business the first mover advantage is a vry well known concept. Being among the first that can supply something will mean that you can quickly grab a large market share and probably get some strong profit margins to boot.
When is this relevant in WoW?
This is relevant whenever a new item or a new recipe is added to the game. You can either go farm the item or get the recipe to sell crafts. In both cases the price will usually never be higher than when the item is just released.
This obviously happens quite often, usually when a patch or expansion hits the servers and everything is changed up.
Why is it an advantage?
The obvious advantage to being the firste one to offer some new item for sale is that no one can compete with you. If players want to buy the new item, and you’re the only one on the realm that has one, then they will have to buy from you. So you can pick whatever price you want. Of course you have to find a willing buyer, but there are always dedicated players.
The first buyers are always the most hardcore players
The players willing to pay almost anything for a new realm will be players who focus on that aspect of the game in a hardcore way. This will mean hardcore classic raiders who want the Bloodvine set and exalted with Zandalar Tribe as soon as humanly possible. It will contain battle pet collectors willing to pay a huge premium for the newest battle pet.
Over time all of the most hardcore players will have gotten the item. At this point the price will have to be lowered as the rest of the people on the realm just don’t care enough to pay the initial price. You’ll also typically see more competition, with more and more players being able to get or craft the item.
There are two things you need to keep in mind when trying to take advantage of this. You want to get the new recipes or items and get them to the AH as fast as possible. This is the main point. If you get the items for sale then you can actually make gold with this.
Then you need to consider how the market will evolve. Will profits crater or just go down slightly? When a new expansion launches the prices for a lot of item will typically fall quite quickly. For instance on day 1 of BfA the insightful +XP gems were selling for 50k gold per. I had more than 100 on day one just from prospecting some ore. It was quickly obvious this market would be flooded, so I started doing large undercuts to get sales from more players. I ended up selling many more than I would have otherwise and probably making more gold than trying to sell the small ones.
If there are some barriers to entry or the item will remain reasonably annoying to obtain then you can likely make a lot of gold for a long while, otherwise you will want to move as much as you can when the going is good.