I dun goofed

Buy a copy of L4D2 and give it to the winner.

You can close the giveaway, there is a red button on its page.

edit : just saw that it already ended. Buy a copy to give to the winner or just bear the "not received" mark on your profile

Try CAG. Maybe you can buy a copy or key for L4D2 to give it away if you don't want to pay full price for it.

This guy might have a copy. Good luck. You have a week, anyway.
http://www.cheapassgamer.com/topic/324914-h-steam-games-for-trade/

You can either:

• Buy/trade the game for the winner
• Accept to have a non-delivered on your profile
• Contact support and hope they can delete the giveaway since it's your first offence.