Hi.
As a someone who didn't make it, a few things come to my mind.
Skill: That's the most important, IMHO. You have to be good, NO, exeptional, with your hands.
Speed: You have to be able to work with a tight schedule in order to at least break even.
Artistic: There has to be something appealing in your instrument that can't be found on the shelf.
You have to ask yourself: Why would anyone want my instrument for X amount of money when a similar product can be had for for example X/3 of the price from the music shop? With replacing option if it doesn't suit me. Or if I have a change of heart.
Dedication/devotion: Instrument building has to be the priority, a main goal, the sole career plan.
I failed on the last: dedication/devotion.
I had, and still have, a million and then some interests, which all eat up any chances of any one from being profitable. But then again I have a degree and work for someone else most of the time.
Instrument building is one of those activities that I see as great hobbies, but lousy careers. Just as playing for a living or doing sound, or being a stage hand, a roadie, etc. for a living. The glamour, if there ever even was any, wears off REALLY fast if You're not 100% into it, let me tell you.
If You're sure, 100% sure, go for it.
Otherwise, it's a great hobby that can even bring some occasional income.
Regards
Sam