If I have a key

Yes, as long as its not one of certain exploited games. The list of games not permitted to be given as keys is in the FAQ.

Sure, why not?
(Translation: There are no problems with keys here. There isn't a "no keys allowed" rule. As long as it doesn't break any other rules it's all A-OK. :D )

Yay!

Sure as long as the key hasn't been used

Is this from the Halloween promotion? Because I don't think those work on steam

Where did you get the key? As far as I know, keys for Plants VS Zombies will not be steam-redeemable.

faq

of course and you're

If it's from that dental website giveaway, I'm not sure these were Steam keys....

You have PvZ on your Steam account, so try activating the key (it won't let you, because you already have the game - you won't lose your key).

If it says the key is invalid, it's not a Steam key, and can't be given away on here. If it says you already have the game, then there is no reason you can't create a giveaway :D

Yeh i think that is fine, but I would double check by reading the rules

http://www.stopzombiemouth.com/