Selling such is a huge challenge. In essence, you are trying to both sell someone that they need to make some change and then sell them on spending money for your efforts.
Generally speaking, something that would be a "one off" project is best done when the prospect comes up with the idea themselves and you sell them on you doing the work.
My best profit came from developing a base project that many people would want but with customizations. The customization meant that each client got a custom project, but the base meant that I could provide that solution at a tenth or twentieth the cost that other people could. (Unfortunately, the market changed, and that base became worthless. It was a great run while it lasted.)