Hi there,
I, too, am selling my iPod classic. All you need to do is connect the iPod to your Mac, open iTunes and it should appear on the left column under Devices. Click on that and then click Restore. Make sure to check for the latest version. Once it's done, simply eject it without giving the iPod a name.
Before ejecting, note that by default, iTunes will guess a name for the iPod according to the name you have set for your Mac. As long as you don't click "Done", it won't be saved to your iPod and when you sell it, that person will then be prompted to enter a name for the iPod. So just make sure to eject the iPod without clicking Done. Also, once you eject it, don't even bother selecting a language on the iPod itself. Let the person who has bought it select it. Just power off the iPod by holding the Play/Pause button and it will be good to go.