Have you ever wondered if you have a marketable book? Most people have, especially anyone who wants to make a living from writing.
You can pay someone to give you their opinion on what’s marketable before spending hours writing. Although you can do internet searches to find them, I recommend going to the websites of agents you respect. Some provide writer services on the side and would gladly charge you a fee to offer their opinion on if you have a marketable book. Other sites provide lists of respected service providers.
However, the operative word here is an opinion. Aside from some basic book tips, the best anyone can do is offer their opinion. Ask two people, and you will likely get two opinions. Often they may conflict with each other.
Consider all the stories we hear about agents and editors rejecting submissions, based on their opinions that the novel won’t sell. But then after twenty, forty, or even more rejections, it crosses one person’s desk who doesn’t reject it. In her opinion it’s marketable. Sometimes that proves correct and becomes a best seller.
All of this to say, you can ask around and even pay for advice from someone to tell you if you have a marketable book idea. But in the end, just go with your gut and write what you’re passionate about.