The game and item are real. Hell, they may be legit games. The transactions are real. Everything is real.
But the source of the money used to buy the games is illegitimate.
Let's say, I stole $100,000. Now, I can't use this money until I find a way to explain how I have it.
If I wanted to wash it with Steam, I could go to the store, buy Steam cards or gift cards with the cash. Set up several Steam accounts, then use those cards to buy my game.
Now I'll lose some money to taxes, Steam's cut, and paying people to buy cards and games, but now I have $50,000 dollars I can use and no one is going to question where I got it.
You may be wondering why I can use the money to buy gift cards but not just deposit it in my bank, but no one is asking why someone has $100. That money is not traceable to anyone.
It isn't as widespread as it probably would be with crypto (start selling shit game, buy game with crypto, get fiat), but Steam already has problems with people doing this, it is usually just more creative as it is hard to buy stuff with stolen credit cards, but we know from a ton of news that it is very common on grey market selling sites.
So the game item and the transaction are fake, as in they’re not actually there to play games.