Professional? Not me.
As with the others, I put them together as a hobby.
I have no customers except me so I only have to please myself.
I have been asked many times by many different people to build a Pennsylvania Rifle for them. My answer up to now has been "when I retire, and have nothing else to do, I may get into building for people, but I haven't retired yet." I know me, and if I were building for a customer, I would drive myself to meet a schedule, if not my customers, then my own.
I would also consider myself obligated to making it his/her way.
That would take a lot of the fun out of doing it.
Not counting the CVAs, TCs, and a number of commercial pistol kits I've put together or restocked, I am working on my 13th now. Of the 13, I have sold 2, (my first rifle and the matching pistol (both flint)). I believe the only reason it sold was I reduced my price to a rediculous price to get some money for a (close your eyes and jump ahead you squemish folks) Uberty .45-70 High Wall.
I have found that finding photos of a gun I really like, researching it and it's maker (when known) and then actually making a similar gun with my own little artistic features (all PC) is a very satisfying experiance.
All of the hardware that goes into my guns is only the best, made in the USA parts.
I admire those who make their own hardware, but I don't have the equipment to do this. I also figure that by buying only the best, if I ever sell a gun, I know the buyer is getting his moneys worth.