Selling faulty equipment and no exchange

We bought a Cell C modem from Cell C in Balfour Park. It hasn't worked since we bought it. I had our IT Technician looking at my laptop to see if it is a problem with my software or my laptop but he confirmed that it is the modem that is faulty. I spent more than a hour at the Balfour Park branch yesterday. They tested everything they could and finally got to the conclusion that the modem is faulty. When I asked them to replace it, they point blank refused and said I need my purchase slip. Today I sent my driver back with the modem and the purchase slip to replace. Again they refuse to replace and kept the slip and the modem. They requested that I personally come back again because it must be my computer. What kind of service is this? And why do they refuse to replace their broken modem?????