Is it a good practice to build an application that have serial number for user to use that application? (Like most of desktop applications)
If so, what is the good way to save the state that user already purchased the application by having the serial number (stating that SharedPreferences and database file could be easily hacked)?
If not, what is the reason?
EDIT: My country (Indonesia) hasn't been able to sell paid application to Google Play (source), and I haven't registered to Google Play. How should I manage paid application by myself?
Many thanks :)