If you don't know the previous owner, they might assume you shouldn't have the phone because most folks would clean a phone when they sell it..
Try going into the Play Store and when assked to enter your details, you should see an option to Existing or New. Choose New and that should stick for all future transactions.
If that fails, you need to power the phone down then press the poer and volume up buttons together then choose the Recovery option. That sets the phone back to factory settings.