You can also buy the cd-key(dvd-key) without the game for
that's not possible, unless the key has never been used before.
why do you think that's impossible
with online activation you get a limited amount of 5 machines per key and you can deauthorize those machines any time you want to and get your activaions back http://support.ea.com/app/answers/detail/a_id/178
you can also activate the game using the dvd which requires you to have the dvd (or a virtual image) in your drive on every startup of the game
that's only for the solidshield/secuROM DRM. if you want to play online, you will need an EA account, and an unused dvd key. If you try to make another EA account, and run the game, it will ask you to "redeem code" (enter cd key), if you enter the one that you have already used, it will fail, and you won't be able to play online. So if you want to sell your game, you need to sell the associated EA account.
If you don't believe me, make another ea account, and try to play bc2 with it.