There are many "starter, template, and blank" themes available.
The one I'm using has this license: https://www.gnu.org/licenses/gpl.html
Its easier to start from these than "reinventing the wheel".
Question is, can you:
The bigger question is, can someone take your hugely developed and coded theme and just resell/redistribute it after making some small improvements?
I've actually been researching these questions, but I see no clear answers.
What is the best way to make a powerful theme with reasonable freedom, and a degree of protection?