Stuck after preparing iMac for sale

Im selling my iMac and went through the steps in “What to do before you sell, give away, or trade in your Mac”, but after resetting NVRAM I powered off rather than pressing press Command-Q at setup screen. Now I'm getting my "user name" and "guest” options when I boot up but my password wont work so I can't get rid of my user account and get back to the back to the setup screen. HELP!