Formatting the drive does not actually delete your data... to permanently delete data you need to actually have 0's stamped over all the 0's and 1's imprinted inside your hard drive. Boot up from your OS install disk and choose Disk Utility... stamp at least one coat of 0's over your hard drive. It will take an hour or more, then you can install a fresh OS for the new buyer. Apparently it takes 7 passes of 0's stamped for your data to be unrecoverable according to the Department of Justice..
Department of Defense, actually.
But, yeah, 7-pass is normally what I recommend for someone who doesn't have the time for the longer option to finish. That won't render it unrecoverable for someone really, really,
really determined with enough time, computers, and money -- but it will make it next to useless for 99.9999% of people with access to the hard drive.