I think that adding "-ist" to a person's interest as a label suggests that the interest defines the majority of the person's activities or attitudes.
Attitude: following Buddha -> Buddhist. (primary attitude, spirituality or religion)
Activity: making art -> artist (primary activity).
(The two sometimes blur together.)
By that paradigm, a martial artist would be someone for whom
a major activity would be the practice of martial arts. This would definitely be true if it were a person's sole activity outside of work. In other words, this criteria seems to be based on time commitment and primacy of focus. I think that making one's living from that activity would be sufficient, but not necessary for the label to apply.
For me, just possessing a high level of skill wouldn't fit unless the person also committed a lot of time to the practice. Sure, the person would be good at the art, a skilled practitioner, but I see applying the label as requiring the
time commitment.
My friend is a physics teacher, but he's a damn good piano player, and has done paying gigs. Is he a musician? Compared to those who teach music or write/play it for a living, he'd say "no," and I'd agree. It's a pastime for him. Would I introduce him as a musician? No, I'd introduce him as a teacher ... if a label were necessary.
I make art on occasion, and also study it at university. Am I an artist? I wouldn't say so, because it's not a primary activity for me, and my skill is up for debate. But ... I make motion graphics for a living so, technically, I'm a motion graphics artist. But I prefer to say "I create motion graphics for corporate clients."
Wrinkle: to me, saying "he's a good musician" is different from saying "he's a musician." To me, the former seems to compliment his musical talents, while the latter suggests that music is a primary activity for him, or that he's a pro.
"He's a good martial artist?"
"He's a martial artist?"
So it's complicated.