You can earn gold in World of Warcraft Classic by.
Gathering and Farming – gather ore, herbs, leather, fish; loot items and gear;
Reselling – buy low, sell high;
Controlling the Market;
Buying gold from Overgear sellers.
In WoW Classic, Professions play a major role in both making money and providing those sought-after items towards the end of the game.
One of the easiest ways to earn extra gold is by picking up gathering professions as early as possible — any two of either Mining, Herbalism, or Skinning.
Overall, Blacksmithing is a great profession for Gold-making if you have time to Farm Crafting Materials/Advertise your services on Trade chat.
Alchemists brew various Potions, Elixirs, Flasks, and Oils from Herbs and other, more exotic ingredients.