This is what I wrote to Valve:
So..... First I want to say that I was terribly stupid when doing this trade, and it was my first trade ever. Anyway, this player who I am reporting scammed me of my CS:GO knife. I had a bayonet knife, and this player pulled a quick one and I lost it. Below, I will provide evidence of the scam using pictures. Sadly, though, I lost all proof of the chat because I quit steam in a fit of rage.
http://imgur.com/r1ZG7P4 - Alright, here is a picture of me starting the trade off. I was going to purpose all of those items, including the knife, to get that one m4a4 Howl.
http://imgur.com/G7GLO7y - After a little while of chatting, he sent me a trade invite and told me to accept. I instantly thought that he meant that he was accepting the offer. I accepted it. After a tiny bit, though, I realized that he had switched out the M4a4 with another gun.
http://imgur.com/IKXMwOR - Once I realized this, I begged him to please return my knife to me. I sent him a trade offer, but he didn't accept it. Instead, he sent me another trade offer.
http://imgur.com/ZzUWLxC - I glanced over the trade, and everything seemed alright to me. I then clicked accept offer, and he got his gun back and I got all my guns back as well. At this I was very happy, but I then soon realized that he had not inclued the Bayonet knife with the trade. Now, he had gotten his gun back to him, as well as stole my knife.
http://i.imgur.com/7pikEay.png - I checked his inventory, and as you can see in the picture, there is my knife.
If there is anyway that you could get my knife back to me, please either contact me via steam or via email. By the way, if in case you need to know, the time the trade happened was August 12, 2014 at about 6:30 PTS. Also, my Steam name is ***************
Thank you for your time, and have a wonderful day.