The argument that I find most persuasive, and that as a fellow game developer resonates with me the most, is that marketplaces like G2A that traffic in stolen goods erodes the perception of value for games. Why would a consumer pay full price for your $25 indie game when they can purchase it for half that on launch day? At best G2A and its partners are exploiting an arbitrage opportunity around regional pricing. More likely and I suspect most often, they are front for money laundering operations that exploit credit card fraud victims, developers, and game enthusiasts who might have their keys revoked.