I will buy an extra copy from someone, please.

hello guys, I sadly missed the last sale for Insurgency Sandstorm, which was a few days ago. im willing to buy an extra key from anybody at 15 US dollars.

Thank you so much and see you in the battlefield.

I don't have anything for you.

I checked out isthereanydeal dot com, and the lowest it's ever been on sale is $15.
By offering $15 you are not providing a very strong incentive to purchasers of the game to give up their extra copies.
Someone would have had to have bought the game on sale at its lowest price and then decided some day (?) that they just didn't want it anymore.
The website also pointed out that it's available for $15 on some website I've never heard of, so maybe that's up your alley.