Tried to sell a demo gas hob unit as new

Did an online order for the first time at builders warehouse and never again. I decided to buy a gas hob and used the online system as they offered an extra 15% discount for first time users. Received a call from the Southgate builders express store to say my item was ready for collection. Got there and when they brought it out the box was open and pieces for the gas burners were missing and the item was also scratched. They wanted to give me a refund but only for the discounted price which meant that I would need to pay full price in-store somewhere else. I asked if I could pick one up at another branch instead but they said their driver would have to fetch it and I would have to come back again when he got back. This meant another 50km round trip because they tried to palm off a demo unit instead of ordering a new one in the first place. The lack of customer service was mind blowing, no offer to try and get a better solution. It would be much easier just to walk in the store and then you can see what you are getting before you buy.