Stolen Apple ID
I decided to sell my iPhone7 Plus. One customer texted me on Viber. She decided to buy my phone but asked to enter her Apple ID to be sure that it works and asked me to send screenshot. I did it (yeah, I know how silly I am). Than she texted me that she can block my iPhone and she wants money from me. I have a phone, I have a box, imei, receipt but I can't use my phone. How can I reset her Apple ID from my phone? What should I do in this situation? What will be if she really lock my phone?
iPhone 7 Plus