Dunno where you can sell it hacked, but you can completely unhack it and sell it without any mention of hacks
If you used "Brazilian method"/browser exploit: just format the system, it'll delete all traces of it
If you installed a Custom Firmware/ Haxchi: look in the guide you used, you'll find the steps to uninstall it on the same theead. DO NOT DELETE HAXCHI IT'LL PROBABLY BRICK YOUR WII U. Be careful and follow the guide word for word.
Good luck dude
EDIT: you've probably posted in the wrong subforum, ask a mod to move this to the ask subforum, it'll probably get more attention there