Q: "...Art is to be shared for free or for sale?..."
A: In my opinion, selling simply adds more value to your work, that is material value, but more than that, your work will have a high value when you can share it, and make others happy, even without people buy it, that's the true value of your work.
However, if you want to continue working in a long time. It is also true if you should sell your work (make the criteria as: why I sell my work? to whom I would sell? which work I can sell? at what price? and what benefit you can get if you sell to that person?).
To convince you, think like this:
if you get the money from selling your work, just imagine what you can do with that money? You can create new works, you can give it to children at the orphanage, you will still be able to share with others in other way.
you always have a choice. Do what makes you comfortable...:)