bought modified iPhone 5s off gumtree?

i just bought a second hand iphone 5s gold off gumtree about 5 days ago and the touch ID didn't work when i tried to set it up so i took it to Apple and they told me the whole thing has been modified so that it isn't even considered an iPhone anymore. When I went to buy the phone, i had a look through the phone and everything was working perfectly, but when it got reset, lots of parts on the phone stopped working? I asked the guy for a refund but he said no. What do I do? Also, since it has been modified, Apple cannot fix or replace the phone for me, but when they opened the phone and had a look at it, after they gave it back to me, I wasn't able to call? I can't hear the other line and they can't hear me? I took it back to Apple and they inspected it again, this time when i got it back home, i turned the phone on silent, and the whole screen turned into black and white stripes! I am so shocked i dont know what i should do next....