Anecdotally, I’ve bought 3 keys over the years from g2a and 2 of them immediately didn’t work. Iirc there’s a big button you click during checkout if your key doesn’t work and the seller immediately has to provide you with a working one. That’s not g2a though, that’s just the seller providing you with another cheap key from their collection. G2A is scammy in other ways too (I’ve yet to be able to cancel their $2 “insurance” fee or whatever they call it the first time, it’s been years and I’ll probably have to chargeback since their site just throws me errors when I try to cancel. PayPal won’t even let me cancel it from their end.)