I get this question so often, and it’s almost impossible to answer with a short answer. So let’s take a look at what goes into it.
We play the same game
Every realm plays the same game. We have access to the same instances, material nodes and quests. So players typically have the same kinds of goals on every realm, and the same access to various items. This means that the same item will usually be in demand on every realm, and most goldmaking methods will work well on most realms.
Competition and demographics will vary
The important difference between realms is the players. What do the players on your realm care about? How hardcore are they? How many care about goldmaking? When do they buy items, when do they Post items for sale? All of these questions influence how and when markets are profitable.
Population size is the most obvious demographic difference. Higher population realms usually have lower prices in retail WoW, whereas in classic it’s usually the other way around. Either way the population size will have a large effect. Larger servers are also usually more hardcore. The biggest guilds attract attention and crowds of people who want to be on the same realm as the familiar names from twitch or youtube.
There is a lot of gold going around on a high population realm, but it’s much harder to extract. On a lower pop realm you will usually have less competition, but sale rates will be lower. I’ve had success on all population sizes and I think it matters less than most players think.
How do you learn all this?
You play the market on your realm. No one can know when your competition posts, exactly how large the alchemy market is or anything else without actually playing there. Focus on items that are useful and fit in with the kind of server you play on. For your competition you will just have to experiment with different posting times to see what effect it has on your sales.
There is no other teacher than experience.
What matters is learning your realm
All realms will have strong goldmaking niches. Focus on enjoying the game and finding methods that work well on your realm, they will exist. Thinking too much about whether your realm is “good” for goldmaking or not is pretty much a waste of time, just test things out and learn what works!