Taken for a ride

I saw bobshop selling a STC3008 which is a unit that switches heat on and a cooling system when needed, I wanted to use it for my Vivarium, I got the unit after collecting it from the seller, I started using it, only to discovery a few days later that the two sensors which are suppose to work independently of each other weren't, I whatsapped the seller and I have enclosed the conversation, now the seller is saying I have physically damaged the unit which I completely deny!!!