title: Android In-App-Purchase, how to check if user have purchased one item
body: <p>is that OK and safe to set a value in SharedPreference to flag that the user have purchased this item? What if user hack this value in SharedPreference. Or I need to connect IAP service everytime to check that before user can use it?</p>
  
  <p>(1) What is the best practice when I use Google Android IAP V3?</p>
  
  <p>(2) And also if user's device have no Google Play installed, I may want to use paypal to make the payment, but how to track the purchase and unlock the features for users if I ask user to use simple paypal payment to get a license key? I do not want to use any other billing SDK, if with Paypal web page to buy the license, How to implement this?</p>