would not exchange product

I bought a mini oven (essentials) from shoprite pelikan park in september it worked fine for a few months but started to trip the power at home.I returned the oven to the shop but they refused to exchanged the oven i was very dissappointed. They informed me the reason why they could not exchange the oven because it has been abused because the oven had some oil on it i strongly disagreed with manager`s view and contacted their customer care line, after customer care spoke to the manager at the branch i was told the product could not be exchanged. I was very dissappointed and felt the oven should have been exchanged as it was still under guarantee