Assuming that you are in the UK, under the Sale of Goods Act, up to six months from time of purchase, it is up to the retailer to prove that the product isn't fault rather than you prove that it is.
Having said that, I have had a breadmaker for about ten years and the non-stick surface is still pretty much pristine and so I am at a loss as to how yours could get scratched under normal use.
I'm afraid that I would take the same view as the retailer - miss-use.
Vic, March 2012