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.
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PS: And FYI, Fontself does not upload your font files to the cloud either.
You're the king/queen of your own castle!