In my mind, a professional anything, is someone that is good enough at what they do, that they can charge money for it, and people will pay it.
Build a model for someone, and get paid for it...to me makes you a professional.
However.....a Professional Modeler can mean different things to different people.
I was once accused of being a professional modeler because I own a hobby shop. Last I checked, just because I owned the place...it didn't all of sudden make my skills that much better than I can then charge money for my finished kits.
I have done some work for a friend of mine in the US....he put out a couple of sets to backdate the Tamiya T55 to a T54, and I build him a couple of kits to show off his conversion parts. So the kits were only assembled, no paint, putty, etc, just the bare plastic. In return I got a couple of free kits and some conversion sets, and some would say because I took payment for what I built, that I should consider myself a professional modeler, I think otherwise, as I don't view it that way. I did a favor for a friend, and he gave me a few items in return for that favor. No real different if you went to a friend's house to help build him his deck...you are doing it as a favor to him, and to repay you, he buys you a case of beer.....does that make you a professional deck builder/carpenter? I think not!
At out show last year there was some question about one of the ship guys. He co-owns a resin ship kit company. He has been accused of being a professional modeler, because of it. Mind you he creates the masters for the kits, and builds one of each of the kits as a finished product to show his customers how the kit can be finished, and what it looks like once complete, in order to sell more kits....does that make him a pro? In my mind no...as he isn't actually selling the finished kits to someone....just builds them as an example of that the kit can be built into. However to some he is blatantly a pro modeler, and complain when he is allowed to enter a show (even despite him not winning best ship! figure that one out).
So...long story short...there is no easy answer, as many people have different perceptions of what constitutes a professional modeler.
Some would say that Ken Abrams is one, (not me) because he is lucky enough to have his work published in a magazine and gets paid for the article. Me? I just say he is a lucky SOB! LOL