Activation lock

I bought an iphone from someone without checking bout it when I tried to activate it, I was at activation lock and that iphone registered by someone else. I contact that guy where I bought it even up to this day he was blocked me. I contact to apple support they need the prove of purchase receipt and they said me lied cuz I don't have the receipt. I'm sad cuz I wasted a lot of money n can't use that iphone and then apple support cannot do anything bout that