Here is a list of CD Key sites,
Green Man Gaming
https://www.greenmangaming.com/
GOG
https://www.gog.com/
Origin
https://www.origin.com
Humble
https://www.humblebundle.com/
GamersGate
https://www.gamersgate.com/
Playstation Now
https://www.playstation.com/en-us/exp...
Direct2Drive
https://www.direct2drive.com/#!/pc
Itch.io
https://itch.io/
Uplay
https://uplay.ubi.com/#!/en-US/

you missed the most obvious one steam