Any help would be appreciated, although I realise I've pretty much fucked myself

So yeah, not too experienced in trading blah blah blah... So I had some TS Essential Accessories, pride and joy. Then I get added and he offers me DOTA 2, now I know DOTA 2 is a popular game and type it into the steam store to see the price of "£22.99", So I think, Holy shit thats a great offer. I accept it, without realising what I had just done. After going back to him an hour later, he was still online, I message "youve scammed me", he immediatley removes me. This means I have basically just given away an 8 key item for a game which is free to play. The point is, he knew what he was doing. https://steamhost.cn/steamcommunity_com/profiles/76561198041136037?l=turkish / http://oi46.tinypic.com/ofzode.jpg