I heard all about people selling accounts and all kinds of black market things.
Steam, Origin, etc.

Are there any stories about people selling their GOG?

Let the legends flow, and the cautionary tales be told!

You might want to reference that year when fallout was given for free. Suddenly all the spare codes had an expiry date - due to people selling accounts with it on.

of course you could just use the forum search or an offsite search to see just how much of an issue its been.

People sell accounts for everything, but to be fair given the number of torrent sites offering gog stuff it's not that big of a concern in comparison. There's a reason account theft across steam/mmorpgs etc is so popular