There will be creative writers who may scoff at the idea of writing for a market, but it is the practice of the mainstream publishing industry to pour their resources on material that has a good chance of being picked up. While industry observers have noted that developing technologies (such as e-readers, electronic delivery, and the renewed interest in tablet computers) and business models (like print-on-demand) would eventually widen the opportunities for vanity and self-publishers, there still remains the question: Will a sizeable number of people buy this book?
If you're a graphic novelist and you want your work to sell, it would be prudent to take your concept and see what kind of audience you're servicing. This is opposed to choosing a profitable market and tailoring a story for them. The latter case is the realm of commissioned work, or for those who are really into the business side of storytelling, but the former case can help you keep a tight focus on the boundaries within which your story should operate.
The first person to consult, really, is yourself. You, as an individual, are part of a larger market composed of people who share your tastes, intellectual level, social and financial status, and interests. So you need to step back, take a long, hard look at your work and ask: will I buy my own work if it were sold at a premium price? If you've convinced yourself that your work is good enough for you to dole out your own cash on (which can be difficult for the self-effacing, or easy for the self-aggrandizing), there's a good chance that people similar to you would take interest.
Note the words "take interest," as opposed to "make a purchase." Big difference, but generating interest is a good first step for an untested product.
You might ask: but what if I'm writing a children's book? Consulting yourself still applies, because you're asking yourself if the work is good enough for you to spend on for a kid.
The next person to ask feedback from is either someone similar to you, someone who you believe might take interest in your lead character, or someone who's into your story genre. Again, raise the question: If you didn't know me, would you buy this book if it were sold at a premium price? Probe the "no" answers to see if there are any flaws in your work, or to determine a decent selling price. Consult with a few people, and try your best to be objective about the feedback.
Doing this exercise can help you work on your pitch to publishing companies. You can give them an idea on your audience profile.
If you're going the self-publishing route, the exercise can help you with whatever marketing you intend to do, particularly when composing press releases, designing the "official website" of your book, or doing targeted selling.
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