***** water pump, & no return

Went and bought a new electrical water pump for my car since the old one had slight leak but working perfectly, fitted the new one they supp****, and it had a rattle on it and wasn't pumping optimally to such that my car started overheating, went back with it since the invoice states there is a 7 day warranty ,was told warranty is void since it was fitted on the car ,don't know how are you supposed to know if it's working or not without fitting it,was told they can't give me another one after spending R3850,this is some shoddy practice and they shouldn't sell parts they don't know are working or not and take a lot of money like that