If someone changes the email/password when they get it, if Game doesn't delete it, when someone tries to log on, it would say invalid email/password, so just change the password now here:
You must login or register to view this content. and tell the person who buys it to change it as well, and you should have no problems.
However, I am pretty sure that they completely wipe the PS3 of everything, so there shouldn't be anything to worry about

^