Android license question

  1. If user cannot test it, but have right to get charge back, and you use local check of license, how you can inactivate it after user calls payment back? Doesn’t it open abuse vector? Maybe better allow trial and forbid chargeback?
  2. License keys unique? Because it seems to be checked locally according to reddit. And someone shared key with me so I can test app and it worked. Is it normal?