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SCAMMER!
Here's all what happen.
https://forums.steamrep.com/threads/report-76561198107247603-steam-steam-games.183894/#post-482090
He is offering for the "beginners" in the steam trading system, cheap games in exchange for expensive games, making them accept the exchange and after that can not go back. In my case, he offered the game Space Pilgrim Episode I: Alpha Centauri (https://store.steampowered.com/app/429470/Space_Pilgrim_E_Alpha_Centauri/) in CD-KEY format, in exchange for my Steam Gift of Don't Starve Together (https://store.steampowered.com/app/322330/Dont_Starve_Together/). My game is very expensive.