[adzii_nufc]

Alright, will try keep it as simple as I can

All games as you first expected are tied to the owner but more specifically the account. I.E My steam account contains all my games, I can not sell a single game. I can however sell the entire account if I chose so.

There's no such thing as second hand codes for any DRM Platform such as Steam, Origin or Uplay. There are second hand codes for games without a DRM platform that can be just installed and played without needing such a DRM platform to store the game. GOG sell exactly these types of games, naturally they're old games because Steam is such a massive gaming entity. For Example, I could purchase a second hand version of Microsoft Flight Sim X and the code works regardless of use. Whereas my Elite Dangerous key is now forever tied to my steam account.

So: If the game you install is via these options - Steam, Origin, Uplay or makes you sign up to a 'service' whilst entering your CD key, it's locked to that account forever.

Legit places to get codes for games:

CJS-CDkeys
GOG.com
Humble Bundle (Charity donations for AAA titles)
Amazon

Legit places to just buy the game outright (Automatically assigned and redeemed your CD key)

Uplay
Origin
Steam

As above though, the old methods of being able to sell on your PC games are very rare now.