1) Purchase Mountain Lion/Mavericks (if you still have the Mini when Mavs is released) from the App Store on the Mini, then do an erase and install. (if you choose to do this, copy the installer to an USB flash drive, give them the flash drive, and increase the selling price to cover the $20/USB flash drive)
2) Erase the HD and let the customer purchase Mountain Lion/Mavericks themselves. (they may have to bring it to the Apple store to have the OS installed)
3) Securely erase the user account and all 3rd party/purchased Apple apps, essentially making the system a new one. (but this isn't 100% guaranteed to remove cruft from the OS)
4) Bring the Mini into an Apple store, explain that you lost the original discs to the computer, and just need it erased/set as new. They have the necessary installation drives to reinstall Snow Leopard, so they would just reinstall the OS, and there shouldn't be a charge. (this option is best if you live near to the Apple store and have time to do that)
It's honestly the responsibility of your buyer whether or not they want to purchase Mountain Lion or Mavericks, so I would go with option 3 or 4, whichever is most optimal for you.