There are many different models and methods for licensing modules.  These range from honesty systems to activation systems, one-time-use keys with key gens and probably some others I haven't come across.

If you're designing a module for commercial sale, you need to take into consideration the type of module it is, who your target market is, and what your goals are.  Some people provide a subscription based service where you get all modules freely within the subscription period.  Others sell a module which is tied to a particular host installation.  Some people provide source code, others don't.

While that is probably not useful information to you, what I can tell you is that there is no built-in scheme for licensing modules within DNN.  The other thing I can tell you is if you treat your customers like thieves, they are more likely to act as such.  Everyone I have dealt with in the DNN Community has been honest and respectful of the licensing terms.  You should expect the same and make your licensing scheme easy to live with.