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.