How to disable find my in a iPhone 8 when you forget Apple ID

The previous owner needs to log out of iCloud or go to https://www.icloud.com/find/ and erase the device, then remove it from their account. If they won't do that, the device is useless. Apple won't unlock it.

Turn off Activation Lock - Apple Support.     

If its not your Apple ID you’ll need to contact the seller and ask they remove it.

As I posted above, the device is useless. Apple won't unlock it.

Then you got scammed, sorry to say.

You are welcome.