It sounds like you would need to implement a license checking process in your app which checks whether the user has paid for it. This can be done however you like it – pages to show prices, details, microflows etc.
I don’t think there is a readily available module to do all that for you but it’s worth checking the marketplace.
In terms of other requirements you would need to integrate with a payment provider and check with Mendix if your contract allows you to sell licenses to use your app. The latter shouldn’t be an issue but worth checking.
The basic license you can purchase directly. For all the other you can use this link: https://www.mendix.com/contact-us/
See also https://www.mendix.com/pricing/
Regards,
Ronald