Piracy

They let people resell keys "no questions asked" (it reduces their liability to not ask questions). Some percent of the resellers they host use stolen credit cards to sell at a loss, and nobody knows what percent. It's probably depressingly high, but (likely) still <50%.

Some percent of the resellers just buys games on sale, or in a cheap country to resell to expensive countries. It's not uncommon when a game has a plummet sale (a $70 black friday sale for $20) that thousands of copies of the game show up for $30-40 on G2A as soon as the sale ends. Those are (generally) not in any way related to stolen credit cards.