Today I want to take a step back from specific methods to cover a more fundamental piece of the goldmaking puzzle. What are you actually trying to optimize?
Time matters
Obviously goldmaking is about making gold. The important factor to consider is the time frame. When do you want to have the gold you are making. Most of my methods focus on making gold indefinitely and that has a large impact on how much you invest and which markets you focus on. If you need gold in a month to pay for a token that is very different than making gold in 2 years.
Target the bottleneck
The main thing is that you need to know what your most limited resource is, and focus on optimizing the gold you gain for that resource. Usually the bottle neck is either your current gold or your time. If you have little gold then that will be the bottle neck and you need to focus on getting the highest or fastest profit you can on all of your gold. If you are fairly rich then you are more likely to be limited by how much time you can or want to dedicate to goldmaking and you are better off optimizing for the highest gold per hour markets you can be in.
Invest all your gold if you can
If you want to make gold you want to spend as much gold as possible on materials you can craft with and items you can flip. Even if you just make 10% profit if you can do that on your entire gold stockpile that will speed you up. Compounding matters here as well, so markets with faster sale rates will help you scale faster.
If you are new, stick to current expansion stuff
This means if you are new to goldmaking you should focus on end game items and materials as they have the highest sale rate. You need to increase your available gold quickly to reach the point where gold is no longer the main limiting factor. That means making a profit every day so focus on markets that sell every day. Good examples include gems, glyphs, enchants, entry level gear and other things players buy to improve their character.
Late game markets
If you are getting rich then you can consider very slow sale rate markets like flipping transmog. This typically requires you to have hundreds of items for sale before seeing consistent sales. You can kickstart your stock by farming transmog alongside buying it, but any one item will sell very slowly. That being said the potential gold per hour is VERY high so this is a great market if you can afford investing in it.