So, how would GOG implement such a system?
All their games are DRM-FREE.
I'm guessing this re-sale of software wouldn't apply to DRM-FREE games that are downloaded.
There's No DRM; No Physical Media; or anything or that sort here that stops the original owner from LOSING their license and LOSING their ability to play the game they got rid of.
If someone bought a game DRM-FREE for download - he could literally keep the installers; make a copy of the installers on disc(s) for someone else; sell the game; and still be able to play the game.
EDIT:
I'm guessing also b/c GOG does DRM-FREE, your games cannot be sold back to them, either.