These days information about goldmaking and WoW is incredibly easily available. With the rise of reddit goldmaking strategy has never really spread faster than it does today. This creates herd behaviour and means there will usually be gaps to use. In addition to this the fact remains that most players rely on established truths rather than their own analysis or testing to figure out what works and doesn’t.
Competition means lower profits. Overall the more players try to sell in a market the less gold you make. Luckily the fact that a lot of players are just chasing the next easy goldmaking method means they will leave other markets behind. So there will always be empty markets ready for you to pounce on.
You also have markets players just dismiss out of hand. Tons of BfA players don’t believe players would ever pay for Uncanny Combatant gear, but it usually sells decently, if not really well.
Players can’t know everything
No one can know everything. This gets very obvious if you read the wow-related subreddits. It’s very common for players to ask about an area, it can be goldmaking, PvE or something else. The answers are often completely wrong or at least misleading. This is because players just can’t know every part of the game. Personally I know my own class pretty well, and I know goldmaking really well, but I know nothing about a ton of other classes. Typically they will only have first-hand knowledge about a smaller subset on what they do.
Filling the knowledge gap
Since players lack first hand knowledge of many things they will use shortcuts to fill the gap. One is of course to take the “established”, repeated truths as gospel. Some of these are just plain wrong, particularly when it comes to goldmaking, as few players do it, and fewer still make content about it. There are tons of examples of established goldmaking truths that are completely wrong, at least the ones that are established truths in places like r/wow or the mmo-champion forums.
Loads of players used to think engineering was an extremely expensive profession to maintain and level. For me it’s been a fantastic moneymaker with some of the fastest selling items and best profit margins I’ve found in classic.
You can only completely trust your own knowledge
There’s no substitution for experience. Nothing will ever help as much as testing something for yourself. Don’t take anyone’s word for it, verify, learn and get better. If an item seems useful and you can craft it cheaply, go ahead and do it!