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http://www.steamgifts.com/support/new
Create a ticket "[Steamtrades] Misuse of feedback". Explain the situation, with screenshots and direct link to his profile (steam64) and account. Then wait. It will take from 2 weeks to a month because there is only one moderator active and she is very busy. It will be removed - he is not allowed to post negative feedback simply because he didn't like the price (and that is not a scam what he was accusing you, that's sharking).
Regarding what and why happened - it is not new (albeit not popular) form of scam to get free games/stuff.
Look at this. This gave me first a -rep on steamtrades for any reason so i added him to talk. when accepted we talked and he told this: http://imgur.com/vospHjV&BBUkj6G#0 http://imgur.com/vospHjV&BBUkj6G#1
HIS PROFILE http://steamcommunity.com/profiles/76561197962439512/
What i have to do now????