Is there a difference between writing for art and writing to sell? What is it (or what are they)?
I believe this is a vast difference, but I also believe the two intersect as well.
What do I mean by that? Well, I'll give you two examples, each an inverse of the other.
Example #1: When I take a writing gig for a publisher who is paying for a story, such as for the Zombies vs. Robots story I did for IDW, I have to write to the specification that the publishers gives me, regardless of what my art dictates. If the publisher wants 7000 words and I feel the "true" story needs 11,000 words, then I have to save that "true" story for something else and come up with a new 7000 word story instead. The same goes for other criteria too. In fact, I have a publisher who despises first person accounts, so that artistic tool is taken out of my toolbox when I work for that publisher. However, within the constraints of that 7000 word story, my goal is to write the most artistic story I'm capable of creating.
Example #2: For my short story collection Show Me A Hero, I entered the stories fairly unhindered. Almost everything was up to my artistic discretion. Story length, experimental formats, POV, tense, etc. I had a full and ready artistic toolbox from which to choose my colors and brushes (so to speak). However, I also had a little voice of reason in the back of my head telling me that in order for a collection of literary-focused super hero stories to sell to the public, I'd better make the decision to reign in some of the more "out there" ideas and put colors and brushes away for this project. I had to self-limit my art in order to make the finished canvas more likely to be successful.
So yes, writing for art's sake can be freeing, and writing for sale's sake can be limiting, but the two can comfortably co-exist within a writer who strives to write the best stories within the parameters the market has officially or unofficially set.
I'll leave it with this analogy: If I want to write a Shakespearean sonnet, then I can't write a 20-line poem without rhyme. If I do, it ceases to be a sonnet. However, within those 14 lines, I can write the best rhyming poem with ending couplets that I can create.
And that, my friends, it the art of selling fiction (or perhaps the knack of selling art -- take your choice).