In 2019 classic I made immense amount of money from engineering just crafting and vendoring iron grenades. Everyone took mining and the low price of ore made these profitable to just AFK craft and vendor.
Another excellent source of money was target dummies. When you use them, they explode and you can loot them. The fused wiring you sometimes looted sold for 1-2gold each. It was the most free gold ever, the only reason you can’t make millions of gold doing it is that the dummies have a 2 minute CD to deploy.