Just trying to sell my overwatch goty game key that I won from online giveaway. I have the game before I won the giveaway so there's no point to have another one

Yeah. Against the ToS here. You can't do that. Also against the Steam ToS. So illegal all around.

Edit: and admitting you got a key for free will motivate someone to pay $45 for it... how exactly?

I'll get back to you on that. :D

Try G2A, their only policy is "makes monez".

I've seen a person want to trade Dragonsphere for ̶W̶i̶t̶c̶h̶e̶r̶3̶ (EDIT: for 3 games) just moments after the former was no longer free, so nothing will surprise me.

EDIT:
Not for Witcher 3, but for 3 games. But to make it funnier, the guy was actually offering the trade before Dragonsphere became paid, saying that he'll send the Dragonsphere codes after they become paid.
https://www.gog.com/forum/general/gogs_general_trading_thread?search=dragonsphere

Yup, get 8 free codes in return for 3 games. Deal of the century.

Then why did you enter the GA?
Anyway, let's just pretend I won the giveaway and you give the key to me! Everybody wins!