It's weird to answer questions about what I wrote with ...what I wrote.
You create (stuffs) at such a high level that it is worthy of being sold.
So, what does that even mean?
I said: "adjective. "Professional" work is a statement about the quality of your work."
Someone says "Wow, your stuff looks professional" They mean it looks worthy of being sold.
So then, is it the audience that decides since them buying or not will decide whether it's worth selling or not?
No, the store does. Most stores don't take just anything and there is a level of the 'same professionalism' that goes into what stores sell what. We all know the difference between a place that sells anything that a vendor wishes to sell and a place where there is an entry bar. Without even selling a single unit you can get a "Congratulations for getting your stuff on there."
does that mean that the experience would get 'professional' as soon as the author slaps a price tag on the novel?
Do you mean after they've brought it to market? Yes. They would be (should be) able to answer all the questions about publishing their own book. Using their experience about finishing a book and getting it on [insert selling place].
But then where is the difference between the professionals or the hobbyists?
This is a can of worms and in the context of THIS conversation, I would say it's about your ultimate goal and possible goals you have not yet imagined. I'm only advising - valuing advice from someone who has done what you are trying to do.