Short: yes, but the answer to this question is a bit more complicated.
As a rule, refunds are generally handled by sellers, since they’re the ones selling stuff on the marketplace.
Imagine a situation where you purchase a key from a seller, try to redeem the game on Steam, for example, and you get the message that the key is invalid and can’t be redeemed, or that it’s a used key (such situations are super rare, like one in a couple dozen thousand transactions, but accidents happen).
The thing you should do is contact the seller and tell them the key doesn’t work. The seller will then try to resolve the situation somehow, like send you a working key. If nothing helps, the seller should offer you a refund that you will either be able to accept or decline. This is handled through g2a’s internal communication system. A bit of a warning here: don’t accept refunds or any other things outside that system, like on messenger or whatsapp. How long does it take? Depends on the payment method you chose, paypal’s different from a bank transfer, for example. Usually a couple days.
But what if for some reason the seller doesn’t want to help you? That’s highly unlikely, because this would cause you to slam them in a negative review and hurt their business, but I guess situations like that can happen. What can you do then? Can g2a give you a refund instead?
Yup, g2a has this refund policy and customer protection program they called the money back guarantee. The way it works is that when a seller doesn’t want to help you or doesn’t respond to you in 7 days, you can file an mbg claim. There’s a bit of paperwork to do. You’ll need to report the incident to the police. Sounds scary? No worries, all you need is just a paper confirming that you filed such a report, and you usually get it straight away. This is just to prove that you’re serious about your claim. Next, give g2a a couple days to process your request and you should have your money back.
Then again, it’s probably going to be an unnecessary step. Sellers are usually very helpful, so if something goes wrong, you’ll probably get either a working key or a refund.
Hope this helped!