Activation Lock

you will need to contact the previous owner to have them log out of iCloud or go to https://www.icloud.com/find/ My Phone and erase the device, then remove it from their account. If you can’t do that, the device is useless. 

Turn off Activation Lock - Apple Support.      

You apparently 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 recovery your money from the seller.

Apple might assist the original owner with proper proof.

Apple does not get involved in 2cd hand sales.

Read the link Eric Root provided above

Turn off Activation Lock - Apple Support.    

Only the person who initiated the Activation Lock can remove it.

Take all the proof you have concerning the transaction to the local

authorities as they may be able to help locate the seller due to the fraud.

I don't think those proofs will work for Apple. But you certainly can contact Apple and make your case. See these support options:

Get a call from Apple support to help solve your problem using this link https://getsupport.apple.com/?caller=kbase&PGF=PGF63005&category_id=SC0245&symptom_id=23362

Apple ID Support https://support.apple.com/apple-id

Apple ID account security for country-specific and region-specific help https://support.apple.com/en-us/HT204169

Get Support: We’re here to help https://getsupport.apple.com/. Go through the options to find the best support option.

Maybe you don't understand that we that provide support here are not authorized to remove an activation lock. Your only option in pleading your case is to contact Apple Support. Use the support links I provided you.

Apple normally requires a copy of the original purchase receipt with the serial number on it.

None that I am aware of.