Bad service

Unhappy with Goldwagen Ebony park mall Managers, I bought temperature sensors worth as little as R200, when is fitted the water is leaking, but when we put back the old one its sealing perfect, went back to refund it n buy the whole thermostat housing because it comes with that sensor, only to find out the don't have stock around GP for housing, the sales man told the manager that's when he refused to refund me n said they don't refund fitted electric part as it says on the invoice, how do u test or know that something is not leaking or working perfectly without fitting it? This brand as whole lost me as their customer because of R200 I will never buy there again, we go the fo quality but service we get is not good at all