Managing a prop project is not for the faint of heart. R&D, Production, and customer service all require a great deal of time and effort. It's a three legged stool. If any one of these areas fail then the entire project can crash and burn. In many cases individual prop makers will handle each of these areas single-handedly. Sometimes this can be done by a dedicated prop maker; but, more often than not, there will be a failure in one area causing projects to fall apart.
I have seen people try to manage only 1 or 2 of the 3 areas of prop project management and still fail. The truth is that prop building is NOT for everyone. Prop collecting is easy; it just takes money. Managing prop production takes a great deal more effort. It's also possible to do more harm than good when you fail in this hobby–with disappointed collectors and damaged credibility the main outcome of failed projects.
If you are one of those rare people who can do all three: Develop, build, and sell props then I salute you. If you can handle even just one of them without having an emotional breakdown then that's pretty good too.