It's only a hobby if the artist says it is. That being said, I believe it is only a hobby if you would do it for fun. If you're only doing it for the money, or you wouldn't just do it in your free time for the fun of it, it is definitely not a hobby, so you really never know if it's a hobby or not unless the artist says it is, because you will never get inside their head for the real reason they draw.
I'd say, amateur ... commission ... artist? My father has sold paintings but he's not particularly well-known ... I never sold art in my life but I don't feel that selling a work or two would mean much in means of accomplishment. So far I've drawn for myself and I'd do commissions if I felt like it.