General question:
In any field, can an individual build anything he wants for his own use,
but cannot build it to SELL, if it violates another person's patent ?
Patent laws are written to promote and protect business. If you have an idea that you prototype or design on paper, you have a fixed period of time to patent and bring to market. Likewise, once you do that you are protected for a duration of time which prevents other from profiting from your idea.
If you do something on your own and do not try and profit from it then nobody is going to try and stop you, let alone even know about it. Even if I had a patent on widget X, and I see you made widget X in your garage, and just used it on your own with no plans to profit, then no laws have been broken.
Of course I'm not an attorney, but I've taken a handful of patent law courses in graduate school.