Account sale : questions

Hi everyone , i'm just wondering how trade or account sale works.

I mean what is the requiements , do i need email or digit code to ganrantie the account ? because earlier february i bought account from a random dude everything went fine but month later i quit SSO and yesterday i downloaded the game and the account seems to be not working anymore.
((P.s : i didn't make this topic to cry about the account , it's all my fault because i was ignorant xD ))

I decided to buy a new account and the question is ! if it was you who gonna buy a new account how you gonna proceed to the trade (change password and other info etc) and i would like to hear all the possibilty.