ok, if you're just selling the computer and do not care about the os.
boot holding down Command (apple key) + s
this boots into single user mode.
from there you can type
but this is still recoverable, though i have to say, recovering data on an OS X disk after you've reinstalled the operating system is not going to be easy.
but, there is another solution... if you want to zero your drive completely. and i mean COMPLETELY! it will require a
low-level format after this... you can run these commands.
at the single user prompt type
and you will be able to see something like /dev/disk1s2 19640808 12730368 6714032 65% /
it's the slash (/) that is important, whichever listing has that is the one you want.
now, type
Code:
dd if=/dev/zero of=/dev/disk###
replacing the ### with what the df command told you for /
good luck covering your kiddie pr0n.