purchase verification and update mechanism

i am new to themeforest and i almost finished developing my theme for themeforest except 2 topics. first problem is purchase verification. I know how to verify purchase by myself. I need to learn how do it in customer end. for example:

After customer installed my theme it has an admin options page to enter purchase key and verify his purchase.

second thing i need to know is adding an update mechanism. According to the envato’s tutorials it should be done using "Envato Market plugin ". But i don’t know how to use or add it properly to my theme.

I read maybe 20 or more articles about these but still my understanding is not clear.
Simply i am stuck in following 2 general requirements in themeforest.

  1. Purchase Verification
  2. Theme Updates (update mechanism)

I am expecting clear explanation from experienced developers about how to do these properly and clear my understanding about these 2 topics.