Question about CD keys

perhaps which site you bought it from ^_^

Yeah, with few exceptions Steam keys don't expire, so you should be able to create a giveaway for it as long as you haven't redeemed it yourself.

of course you can
as long as you have a working key that (obviously) has not been redeemed or given away

Giving away keys from G2A can be problematic, because there's a definite chance that they will be bad to begin with, or get revoked later. I accept that risk when I buy G2A keys for myself (and in fact G2A has always made it right so far), but I don't put them up on SG.

NOTE for the pundits: Turns out there's also a risk of this with "official" channels like Humble. Bad keys happen. It's very rare to have a key revoked months later, though, and to date those have always been mass purges by shitty developers, which the community is well aware of. (Edit: Aside from that NBA 2K17 screwup, I guess.)