I replace the good
used part and place the used part in my spares, as I know that part works. I
don't know how many times I have taken a part out of the box and had it not
work, be the wrong part, ect. Any comments?

I think testing a spare, if reasonable, makes more sense. Using the original as
a spare just means it could close to failing, like a badly worn spare tire.
What you really need is 2 spares for important stuff. Alien carries two of most
engine and electrical spares from bulbs to engine and autopilot belts. We even
carry two full emergency and med kits.

RB