If you know all the stuff that needs to be deleted just throw it all into the trash. Hit delete.
Open Disk Utility > click the main parition > Erase Tab > hit "Erase Free Space"
Just use Zero Out Data all the other options 7pass or more are just nonsense and provide no reasonable benefit. They exist for the deranged paranoids only.
An easy way to not forget any user files is to create a new user and delete the old one with all its data. That way all caches, mail data, ical data, docs, movies, music is gone in one swoop.
Now it is clean of user files (unless you did some uncommon stuff outside the user folder but you'd have to know about that).
It is not a factory reset as apps and stuff is all still there. A true factory reset needs a full delete and reinstall. If the GPU is so dead that you cannot start the machine and you have no other Mac to do such an OSX install, you can just wipe the hdd and send it to that buyer.
He will need to repair it anyway to get it to work and should install osx himself. Just send the OSX discs along.
You can wipe the HDD (or do all the steps above of deleting user files) by just getting some external hdd drive from somebody. Putting the HDD in there. Plug it into any computer that you can find from family, friends, school, work ... and wipe it.