Noob question about account purchase

Wanting to purchace an account from someone. I am in the beginning stages of trying to play wow. Right now I'm just messing around with my trial account... I want to know if i buy an account from someone that has cataclysm in his account, do i still need to buy a digital purchase from their website or will i be able to download the game for free because he already has it on his account? Kinda confused about that. Also if i don't need to purchase it and am able to download it from the account that i just bought why doesn't everybody just download the game from their friends account...? Thank you.

 

You can download the whole game without needing to buy it first. You need an active subscription + an account with at least vanilla/bc (combined now) to log onto the servers. Basically you have to install all expansions up to and including cata, but can't logon without an account. So yes, you'll be able to download the game then just put in his details and as long as you have game time you can play.