[The Surgeon]

suppose i wanted to sell 2 steam gifts for left 4 dead 2 game , am I allowed to do it and where can I post if yes.

[Hash G.]

It's not allowed.

Steam Gifts
We don't allow selling of Steam games as keys or gifts. You may only
sell Steam accounts in the Non-Free Accounts forum in the Marketplace.

From Disallowed BlackHat Activity

I'd suggest to sell them on http://www.kinguin.net/

[webdev77]

You can only sell the whole account with games already redeemed on it as Hash G. so kindly shared from the Help Docs.

Honestly, reddit might be a good option to try as well.