If I steal your credit card and buy a key to the new FNAF that costs $60, I can sell that key on G2A. Let's say I make $50. That $50 is mine, the third person has the key and thus the game, the developers have your $60.

But your CC was stolen. You shouldn't pay for that. So you contact your financial institution and are like "hey wtf I don't wanna pay for this." They respond "yes sir/ma'am, sorry sir/ma'am" and return the funds to your credit card.

Now your financial institution isn't just gonna take the hit. So they're like "well, this money is with the weird dude who made FNAF, so let's take that money back." They issue what's called a chargeback, reclaiming the $60. But that's not enough, right? Because "We don't want them putting their products in sketchy places that might make us do this more, so we're gonna add a punitive fee." These punitive fees range from $30 to $100 depending on your institution.

So now, I have the G2A user's $50, you have your $60 back, the third person has the key, and the FNAF developers? They had $60, but after the chargeback that turned to $0, and then there was a punitive fee. They now have -$30.

Epilogue:

After losing the money, and in fact paying for someone else to have the game consider their options. They'd be right to revoke the key as the person who used it didn't actually pay them for it. But if they did, they'd get bad reviews, the G2A customer might be mad at the FNAF devs, and you may just go and buy another key from G2A. So in all likelihood they just cut their losses and let you keep the game as it causes less problems that way.