How do I sell my CD key?

I have a DayZ key I want to sell. I've never been banned, kicked, anything. Bought it legitimately (off Steam), it's my first and only key. I'm pretty tired of the game at this point.

Here's my question:

1) how do I find out my own CD key?
2) how much would someone pay for this (it's Arma 2 and OA).

PM me I guess if you'd like to buy it.

Hopefully this isn't against the rules, if it is I'll delete the thread.

How much do you want for it? Got any gear?