Most = prevent.
The issue with G2A is that any keys at all come from scammed credit cards. In a silly way it's like of like tor. It doesn't matter if I am trying to sell my excess Humble Bundle keys in good faith on G2A if other sellers on the market are selling scammed keys. Good users making listings obfuscate all the bad users.
Also, purchasing regional keys cheaper and reselling them is also what causes this shit in the first place. People blame Valve for making the decision, but not the people switching to a region to buy a game for cents on the dollar and then resell it? That is actively hurting the people in those countries who are now being charged closer to USD prices. For Brazillians this is exorbitant.
I don't disagree with you in that there are G2A keys that come from bundles. But I do disagree with the notion that "it doesn't matter." It absolutely matters because it's affecting people's ability to buy games and it affects people circumventing legal purchase methods (of which I support their circumventing) who then have to deal with buying scammed credit card keys instead of me selling them and excess Humble Bundle key. The card gets charged back, the developer loses money, the G2A purchaser loses their key, and the scammers get off scott-free.
Basically, G2A should be a good idea but has been co-opted by scammers. These sites have their grey-market reputation for a reason, because it's run entirely off of the losses of others. Losses of the developers, losses of regional players, and losses of players purchasing games on these grey-market sites.
There's no winners for G2A except for the owners of the site and scammers. You may win once in a while getting a brand new game for $5-25 less. You may end up losing when it's pulled from your account, if it does. At that point, you're effectively gambling. Taking a risk for a discount on something with a high likelihood of it being unethically sourced which may be removed from your account?
In most cases I'd personally rather pay the extra $15 to just have the peace of mind. The chance of the game not being bought on a stolen CC and not supporting regional theft that hurts those players is just a bonus.