Of all homes listed, what percentage do not sell?

[Darrin Howells]

I am starting a real estate photography business and I'm trying to find one very specific statistic.  I would like to know of all homes listed, what percentage do not sell.

I'm sure there is an easy way to find this, but I spent a couple hours online searching and could not find this statistic.  I *don't* need any granularity, like condos vs. single family, or various reasons why the house didn't sell.  Just a percentage like 23% of homes did not sell in 2017.  

Anyone happen to know this off the top of their heads, or know where I can go to get it?  I don't have any memberships anywhere yet (bootstrapping this business so watching my pennies), so public info would be preferable.  

Thanks for any help.

- Darrin