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.