If you erase this disk, they will have to re-register anyway.
If you don't know how to erase, put your install disk in, go to Disk Utilities (in the top menu bar), select the drive, and click the Erase tab. There should be a bubble for Security Options; click this, and depending on how paranoid you are, select the number of passes you want to do. If I were selling the computer and I had important documents on there (which i have backed up), I would probably do the 35-pass erase, but if you are low on time, the seven- or even single-pass should suffice.
Let the new user install the system, and they can re-register it.