CD Keys.
What makes a CD Key go from valid to in use?
CD Keys.
Means somebody is playing on that key on that moment.
More than likely the original owner of the key is playing on it. 
Similar Threads
WarRock - International Hacksobtaining a warrock key
2 General Game Hackinggiving away GuildWars 14days and 10hrs trial acces key
6 General Game HackingNeed Battle.net Key For Diablo
16 CounterStrike (CS) 1.6 Hacks / Counter Strike: Source (CSS) HacksTrade Css cd key 1 WarRock - International Hacks[Warning] alt key=crashMI 3
2 General Game Hackinggiving away GuildWars 14days and 10hrs trial acces key 6 General Game HackingNeed Battle.net Key For Diablo 16 CounterStrike (CS) 1.6 Hacks / Counter Strike: Source (CSS) HacksTrade Css cd key 1 WarRock - International Hacks[Warning] alt key=crashMI 3