If you are really concerned with the next owner recovering your files, which can be done even if you emptied the Trash, then you should format the drive:
Open Disk utility, select the drive, select the erase tab and click on security options, then choose one othe following:

I don't know why the sale doesn't include a wipe and reinstall, but I don't see why it shouldn't as a mac always comes with a licensed OS, you just format the drive and reinstall MacOS and you're done.

PS: Oh yes, I forgot to mention you should do this after booting with the OS install DVD, from the menu option utilities (I think)

If you are really concerned with the next owner recovering your files, which can be done even if you emptied the Trash, then you should format the drive:
Open Disk utility, select the drive, select the erase tab and click on security options, then choose one othe following:

I don't know why the sale doesn't include a wipe and reinstall, but I don't see why it shouldn't as a mac always comes with a licensed OS, you just format the drive and reinstall MacOS and you're done.

You can't erase the drive you are currently booted to. You have to boot from an external drive with Mac OS X installed, or boot from the Install DVD.