Simply reformat the hard drive and reinstall OS X. Boot from the original Software Restore CDs/DVDs that came with the computer, and when presented with an installer menu, select "Disk Utility" from either the "Installer" or "Utilities" menu. Highlight the hard drive, click the "Erase" tab, select "Mac OS Extended (Journaled) and the rest is pretty self-explanatory.
Then, quit Disk Utility, and follow the menus to install a fresh copy of Mac OS X. That way, when the new owner gets the computer, they'll have a fresh copy of OS X waiting for them to personalize how they see fit.
There's no sense in trying to take what's already on there and salvage it, since you won't be in possession of the computer anymore.