And see: Activation Lock for iPhone, iPad, and iPod touch https://support.apple.com/en-us/HT201365See
this heading Check for Activation Lock before you buy a device from someone else.}}
Apparently you were scammed when you acquired your phone second hand from someone other than Apple or an authorized Apple reseller. Unfortunately the phone will be useless to you. The links below explain what has happened. You didn't know to heed the advice given below.
Before you buy an iPhone, iPad, or iPod touch from someone other than Apple or an authorized Apple reseller, make sure that the device is erased and no longer linked to the previous owner’s account.
And see: Activation Lock for iPhone, iPad, and iPod touch https://support.apple.com/en-us/HT201365See
this heading Check for Activation Lock before you buy a device from someone else.
What all this means to you is that there is no way we can help you. We are not authorized to remove the activation lock from any phone. And, under your circumstances, Apple will not help you. All I can suggest is that you try to recovery your money from the seller.