The person you are buying the keys from has probably resold them or if you change your key then run arma 2 oa from steam it will change your key back to the original one that you had, also next time try buying from someone who has a lot of vouches on their thread here: DayZ Selling / Trading / Buying - MPGH - MultiPlayer Game Hacking & Cheats - Hacks, Cheats, Downloads, Trainers, Games