Easier said than done. Your employer would probably prefer you to stay and build it exclusively for them, rather than leave and also sell to their competitors.

There’s also the issue of trust. For some reason, managers tend to trust third parties more than their own employees. For example, they won’t think twice about using open source libraries with a single hobbyist maintainer, but often feel uneasy about using an open source library built and maintained by one of their subordinates. I’m not sure why this happens. There must be a name for the phenomenon. I suspect the same thing will play out when you try to pivot from employee to supplier.