Although the game itself is free, Fortnite also features virtual coins that can be used to purchase various in-game items. Scammers use stolen credit cards to buy V-bucks and then sell them for good rates to legitimate players. Just as expected, these transactions are being done via the Dark Web.
https://www.notebookcheck.net/Fortnite-is-being-used-to-launder-cash.394616.0.html