Bought a 6s & been conned

Hi all, I bought a used Iphone 6's from a shop owner in my town, when i got it home I put i tunes on but it was running very slow. I got back in touch with the lad from the shop and he said it will be the battery. In this time his shop closed down, he came and put another battery in phone, he then went and said it would now be fine. after he went I again tried to put phone on but found out it was worse. I messaged the lad but now his shop has closed and now get no reply from him, he lives out of my town so I cant get hold of him.
The problem with the phone is once I switch it on, I get about 2 minutes of life while connected to the charger before it switches itself back off.
Has anyone got any Idea how to fix this or how much I would get for it as it is ? I paid 160 pound for it and never got chance to use it.
Thanks for your help in advance