I bought an iPhone 6s and received it pretty quick...

I bought an iPhone 6s and received it pretty quickly however, after use I wanted to resell it and I wanted to return it to them. They now told me the phone was reported stolen and that it could not be sold, or returned to me.
Since I bought it from them I can only assume they did it on purpose.
After I brought up that it was their problem they sent me a stolen phone without checking it, they stopped replying.