Nobody calls it "CFW" lol
Reinstall clean copies of any modified IOS
With a title deleter, uninstall any IOS with a number equal or higher than 81, and any non-stock channel - except the HBC (if you know what you're doing you can remove ALL IOSes EXCEPT the one used by the Wii Menu and the one used by the HBC, and the MIOS)
Uninstall Priiloader if you use it
Run the Hackmii installer and remove BootMii from boot2 if you use it, and the HBC
Update your console to get back all those pesky stubs
Factory reset
In practice, if you just want to stay within the letter of the rules of eBay, want to not have a stupid buyer complain, and the console was modded according to best (or decent) practices, you can just remove Priiloader/BootMii with their installers then do a factory reset, which will remove all mods which actually have some functional effect (CIOS is useless if no app tries to load it)
If on the other hand you wanted the academic exercise of removing every single trace, even those that Nintendo may not have looked for in warranty claims... good luck manually deleting custom channels from title.db, ticket.db and uid.sys without screwing up
I don't think the format is even really documented!
