Is this allowed to sell a game for TCs ?

If it's a game on steam, yes, or anything really where you give a code and it's allowed by the company who owns it.

Ok thx, but how to sell one of my steam game ?

You can't sell steam games because you don't own any physical license, I think I read it somewhere on steam support

I thought you had to gifted it

First buy the game as a gift, then send it to the customer.

just putting it out there, if you're selling that something that violates the terms of service of the game/distribution company your thread will be closed and you may be infracted.

I believe he says "one of my game"
Thus, it's not possible.