Updated November 1, 2018
Answer: "Great Art Sells"
This from an art dealer that I met years ago.
And although her words can certainly be argued with, I've never forgotten them.
While it's impossible to define great art, what I believe she meant was that artists unusually sell their strongest work, and that subject matter is secondary.
Whether they know it or not (and they usually don't) collectors respond to the way a work of art makes them feel, more than they do to the content. And that includes the other intangibles, such as the artist's level of inspiration, commitment, and passion for their craft.
I've learned that people will forget what you said, people will forget what you did,
but people will never forget how you made them feel.
~ Maya Angelou
Every gallerist I know says the same thing:
Don't paint for the market. Paint what you love. Show only your strongest work.
If you're showing multiple pieces, then don't expect the stronger works to lift the weaker ones up. Expect the opposite; the weaker pieces will pull the rest down.
The 80/20 rule, helpful in so many situations, applies here: 80% of the work that you do is for artistic growth; 20% is for show.
The best part about this strategy is that it leaves you with a nice archive of work. (80/20 applies to studio time as well; 80% observation / 20% execution).
I know little about the commercial success of the great Giorgio Morandi. I only know that I saw the piece featured above, in a small gallery, over a decade ago. It's stayed with me ever since. I can see it with eyes closed; I can smell the dust on the table.
Making an impression is one thing, connecting is something else entirely. And that's what the game of painting is all about - connecting. Taking someone out of their reality and bringing them into yours, through your art. It's a magical, liberating feeling.
If someone can describe a specific piece you've done, along with the feelings it evoked, rather than have a vague impression of you as the artist who does "such and such", then congratulations! You've hit a home run.
Hit a few more like that and you're well on your way.
Here are links to a couple of bloggers with wonderful insights on the subject: