It's a thing people do in any game with trading.
They exchange a valuable item for a worthless one, and then afterward, the party trading the worthless item sends some PayPal cash to the person who sold him the valuable item. In games like Counter Strike: Global Offensive and Team Fortress 2, it's not unusual for players to sell items for hundreds (and occasionally, thousands) of dollars.
You can imagine how common scams are in systems like this. People avoid them by using middlemen and only buying/selling from reputable dealers, but it still happens.
The players with the most valuable items in games like those are the ones who paid the most real-world cash for them. It's not something that can be avoided.
While players like me could potentially make a lot of money by selling items, I feel like the legitimacy of the current looting system would be too badly compromised by the threat of people doing this.