It is well known sites like G2A sell stolen keys and IMO that is worse then pirating games. You can't prove a pirate would have bought said game. But you can prove sites like G2A sell stolen and ill gotten keys. Stolen and ill gotten keys is a lost in profit for developers. I am sure someone will bring up the used games argument but the difference is once again you can't prove someone would buy the game new and to get a used game it must have sold new in the first place.
Source
http://www.polygon.com/2015/2/9/8006...ket-game-codes
There is many other sources that report on this as well. Developers receive nothing from a G2A sell. The sites don't even protect there customers unless you pay extra (No matter how small the amount is).
So IMO people shouldn't be allowed to promote said sites on here.