Faulty Item

I bought a car amp from TV World on the 11th of August 2015 and took it, along with my car, to Quadrophonic, a professional, to install it. Unfortunately the amp gives problems every time they connect it. I returned the amp and asked for my money back, but the owner at TV World refuses to give me back my money, he wants to send it in for repairs. I feel this is not right as the box i got the amp in was not sealed in any case, so it could be a faulty one to start with and now he blames us.