Goldwagen Welkom. Selling faulty spares

I took my bmw to Welkom to service center. They checked and found water pump were leaking. They phoned Goldwagen Welkom, got a new water pump with a new housing and a new thermostat. It took like an hour and Goldwagen brought the spares to the service center. I were standing with the owner and mechanic at my car when they fitted everything. When filled with coolant you could see water dripping from thermostat. They removed it and you had to look very carefull to see their is a fine crack in the thermostat housing. They called Goldwagen Welkom immediatly. That whas like half an hour when they deleived it to the shop it were found to be faulty. They sent the same person to come and picked it up their. But now you have to buy a other one and pay for the new one and wait for feedback from them. It has been like 2 weeks now. Now goldwagen Welkom phones me, i can come and pic up the thermostat and report but they wont be refunding me. How do they get away of selling faulty products? I've supported Goldwagen over the years. But receiving faulty products from them to the service center and like they were phoned back like in half an hour time and reported that the thermostat were faulty. Just to tell you sorry they wont be refunding me. Be carefull, i see now the other review's. It looks like they sent out faulty parts with No regard to their customers