Can't you just drag all your files to the trash? You can sift through the Preferences and delete those too. I always keep all of my files in 3 main folders (Favorites, Music and Photos) so deleting those would clean up most of my hard drive. This won't create a new administrator account but the new owner could live with it or find their own OSX install disks.
Alternately, you should have install disks for the iBook. Are those locked to a specific hardware? I know I used to use software disks between computers, but maybe they do a hardware check these days. If not, just erase the disk and reinstall the iBook system.