What you're seeing is factory reset protection -- it's a security measure. You would need to ask the previous owner to help you sign in with their account, and then afterwards you could do a proper factory reset (which would also remove their account correctly). If the previous owner legitimately sold you this phone, they should be eager to help, because they should want to make sure their account is off the phone. But if they ghost you, then there's a good chance it's a stolen device.
I'm sorry, but we don't allow discussions about trying to bypass security measures like this. Hopefully you can get your money back.