all this fuss
if its needs to be wiped then do factory reset from menus, also deregister it on your amaon web page.
then to prove it is wiped, connect it via USB to PC ( do not re-register it or log in) and confirm with file explorer that there are no books or docs on the drive.
if that fails, try a hammer
ps from memory , de-registering it at amazon in order to sell it does not remove side loaded books, i.e. its possible to sell a kindle with some books on it.
but factory reset does remove ALL local content, leaves it looking like it did when it left the factory - hence the name
reading his, and the previosu posts, does suggest that flash memory has failed and is not delete-able. so accept that and move on, or take a hammer to it
https://www.amazon.co.uk/gp/help/cus...deId=201734230
the option for newer models is now called reset ( not reset to factory)