You could sell a database for filemaker users (multiple users)
You sell a database for iOS Go users (single user)
You could sell runtime databases created with FMP Advanced (single user)
You could sell Templates for beginner FMP users.
Security can be done with passwords, hidden scripts, timed trial periods, authentication online, and other methods.
You may want to do some Google searches. (adding site:forums.filemaker.com would limit the search to this site)
And maybe join the Filemaker Technet Forums (it's free) https://fmdev.filemaker.com/login.jspa