Place the hard drive in another machine, with some disc scrubbing utilities is a good way,
Or you can use file manager to delete every thing, reformat the drive, then defrag the drive. format and defrag again a few times.
The drive will now be ready to put back in the original box, and reload the operating system.
If the buyer realy wanted to, he/she could take the time and trouble to recover a bit of the previous information. They would realy have to want to see your old emails or what ever to try.