Try this:
boot to diags (directions included in the serial port jb thread)
use the diags menu to export the USB storage
USB cable, Kindle to PC
put a file in the topmost level of visible USB storage, named:
DO_FACTORY_RESTORE
All caps, no extension, no content
safely eject Kindle
remove USB cord
re-boot Kindle.
That should fix it (and would have fixed it for the person who sold it to you
).