As long as you hold all rights for the original designs you create, you can do whatever you want with your fonts, including selling them ;)
Have a look at what other creatives have done with Fontself: https://creativemarket.com/fontself/collections/273597/Made-with-Fontself
You can learn more about usage & rights in our End-User License Agreement.
PS: And FYI, Fontself does not upload your font files to the cloud either.
You're the king/queen of your own castle!