I am sorry about your situation. 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/HT201365 See 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 since you cannot provide proof of ownership that Apple will consider to be sufficient. All I can suggest is that you try to recover your money from the seller. I know that doing so usually is impossible though.