If you know what players are planning to do with the items you are selling you will be able to sell your items quicker for more gold. If you can cater to them directly.
Make it easy
The idea is to make it as easy as possible for the buyer to use the item or items for whatever he is planning to do with them.
Knowing what your buyers plan to do will also help you set the price at the right level, as different players will have different thresholds they are willing to buy an item at.
Making it easy for players to get exactly what they want will also make them willing to pay more. People are always willing to pay more to avoid hassle.
Price level
First let’s consider what different uses will mean for the price level. One obvious example is for materials. If you are selling materials to players that are planning to craft with it to sell the finished goods, then they will care more about the price than players looking to level a profession.
So generally you will be able to sell materials with more of a “I need this to get the profession” for much higher prices relative to their market price.
As a practical example my material flipping operations are based on selling to crafters that want to make a profit primarily. To accomplish this I sell in large stacks at prices very close to dbmarket as they will typically always be willing to buy large amounts of materials at that price level.
Stack Sizing
Stack sizing is one of the main ways you can zero in. Selling items in the exact stack size people need or want will help you get a much higher price than you would otherwise.
This obviously only works for items that can stack, but it is very profitable. I used this extensively when selling flasks in legion. I would post in stacks of 2, 3 and 4 at prices that were relatively high. These stack sizes represent the amount of flasks someone would need for a typical raid night.
Just try to figure out what stack sizes player want the item in and that other people are not posting, and you can usually get a better price.
Summing it up
Always consider: “How badly does someone want this?” and “How many do they want?”. If you utilize the answers to those two questions when posting your stuff you will get more sales and you can get better prices.