Issue with deleting user account
To add on to the existing answers and offer a different solution, you can securely erase the drive instead of just one user account.

If you want to sell your machine but also include installed software, there is a better way to set this up that will ensure none of your personal data is on the machine, as well as provide a better box opening experience for your buyer. 


I would suggest doing a secure erase of your hard disk first. 
Then, reinstall your OS. 
Configure a dummy user account named "test" or "admin" or something generic
Install the software you want to include
Restart your machine in Single User mode
Very carefully, do the following:



  mount -uw / (This mounts the root volume so you can make changes to it.
  
  rm /var/db/.AppleSetupDone  (This removes the flag that tells Setup
  Assistant that it's done)
  
  rm /var/db/dslocal/nodes/Default/users/"shortname".plist (Replace "shortname" with the user account name you set up before. This removes the user plist)
  
  rm -rf /Users/shortname  (Replace "shortname" with the user account name you set up before. This removes the User directory)
  
  rm -rf /Library/Preferences  (This resets preferences)
  
  shutdown -r now  (This restarts the computer)


Verify that your machine starts up to the Setup Assistant and you're done. That way, your buyer can start up the machine to the Setup Assistant and still retain the software you want to provide them with. 

Good luck.