JMich: G2A is a store, and a marketplace. So you can sell your games there, and they do seem to sell retail codes from boxes bought in different regions, but the marketplace part means it's a lovely place to unload codes that will be revoked down the road.

Think of it like buying a car in a county festival. You may get a good deal, or you may have someone else's car.

JMich: Retro City Rampage had that option from the start from what I recall, so, October 2012?
And this is not reselling. This is selling. RCR is the publisher selling codes, while Amazon, uPlay and Origin are stores selling the codes the publisher gave to them.
Those are not gift codes a user bought from GOG and sold elsewhere, those are bonus codes GOG gave the publisher to do with as they please.