Ask the seller to try the links below to recover their Apple ID/password. They can also contact Apple ID Support. If they refuse, ask for a refund.
Apple ID - How to find your Apple An iCloud e-mail address is an Apple ID.
Enter your Apple (iCloud) ID and click Forgot Password.
If necessary, there is a contact link.
Apple Support by e-mail or chat Select your device, Battery, Power and Charging, then Power adapter not working to get a chat option. If you need to talk to them, ask them if they will do a Skype or FaceTime with you.