I made some money in Diablo 3’s auction house when it was still a thing. The process was simple. The game-gold auction house and real-money auction house sold the same products at different prices.
I focused on gems and to a lesser extent gold. Gems were the main focus with gold being more of a currency of convenience.
The exact mechanics of the auction house are hazy to me now since it’s been a few years, but you wanted to, say, buy gems on the gold auction house to sell on the real-money auction house. Then you’d want to buy gold on the real-money auction house to purchase more gems on the gold auction house.
Some days you’d want to switch it up, but you could just keep flipping back and forth between markets. Different gems would also come in and out of demand, paying higher premiums or selling at discount. Most days you could also buy lower-level gems, combine them, and sell the higher-level gem for a profit.
The money wasn’t great or anything, but it was fun while it held my attention. I played it a bit like how people played cookie clickers — a few minutes every so often.
I also did similar things in Discovery Freelancer and Eve Online, being a space trucker, buying items in one station and selling them in another. God bless autopilot.
Economies are a nice little value-added in games. After beating the game, you can still play the market.