I thought it was obvious at this point that a lot of G2A sales are "fallen of truck merchandise".
Note: For this part I do not have a source myself but TB mentioned this on his podcast so i assume it is true.
Apparently once they got stolen keys and people are getting screwed over they contact the developer and say to them that they (the developer) has to use their service to fix things, otherwise people will refund their and the dev has to pay.
Yes, either the developer uses this "service" and has to pay money to G2A to fix the crap that G2A caused in the first place, or the developer has to deal with the whole situation themselves including refunds which can cause them a lot of money.
This is essentially the equivalent of a mafia group demanding protection money or something might happen with your shop.
The worst of all is that G2A actually sponsors a lot things like streamers so there is a big chance when you get to a popular streamer there is a g2a link in the description for stuff.