you cant. you need to buy your own personal unused copy.
Sins of a solar empire does not require a cd key to install. Does not require a cd to run the game but it DOES require a serial key to play online.
The money you spend on the game when you first buy it is more like spending a money so you can play multiplayer.
Whoever sold you the used copy was smart because with just an account created from that serial key, he/she can now just download the game from Impulse and play. that person doesnt even need the cd no more.
