A comment I have gotten and seen several times over the years is “Why would anyone buy that?”, usually in relation to some actual sales in some market. The answer in these cases is often yes of course. And trying to figure out why someone would want an item is a great help in figuring out what to sell.
Your way to play the game is not the only way
This is the main point of this entire post, and I could probably have stopped here. Just because an item doesn’t fit into your goals, or seems bad to you does not mean that it can’t be crafted and sold profitably.
Rendering judgment without actually trying to figure out someone wants an item will keep you from finding new markets and is a major hindranc.e
How can you avoid it?
Ultimately you need to figure out how others play the game. You can do it by reading forums and seeing what people talk about, by reading guides and by talking to your guild mates.
You can also do it by just thinking about what sorts of items would help you in the activities you are currently doing. As an example when I was leveling my second character from 110 to 120 I wanted a socket for the experience gem. I figured out tha the simplest guaranteed way is the level 111 crafted uncommon rings, as they always have a socket. So I crafted one for myself, and 10 for the auction house, and they sold as other players also need sockets for their experience gems.
There’s more gold to be made than you might think
There are a lot of things to do and collect in WoW, and there are some people dedicated to a very wide variety of goals. You don’t even have to know anything about a niche to make gold in it. I know next to nothing about pet battles, but I still sell a lot of battle pets. I just rely on fairly objective price measures to find good deals. The price already tells you if a pet is desireable. All else being equal a higher price means more desireability.
I considered making my point one more time, but I honestly cna’t find a novel way to write it.
Focus on how other people play the game, then find items that will help them reach their goals. They will be willing to pay, even if you would not be.