言っとき【いっとき】 is a colloquial contraction of 言っておき, where this おき is the masu-stem of おく, which is a subsidiary verb that adds the nuance of "leaving the resultant state". See this question, this chart and this article.
This 人のこと is closer to "things about someone" rather than "things to people" (see this). This 言う is using the
A + を + B(だ) + と + verb pattern (see this and this). Note that this と is not a nominalizer but a particle close in purpose to English "as". Thus 人のことを魔法使いだと言う effectively means "to call someone a witch", and the 人 refers to "me" in this context. A literal translation would look like this:
While having called me a witch or a monster, he...!
He's said I was a witch or I was a monster (and has never corrected that), and still...!
The part after ながら is left out, but something bad about Goku is expected. (Perhaps something related to his ignorance about his own abnormality? Depends on the story.) That's where "had the nerve" came from.