Protocol not followed!!

I for the first time made a purchase from the PEP cell store in Boksburg, I bought a memory card. When inserting the memory card the phone said it was *******ed, I took it back the next day. Where they refused to exchange it, saying I damaged it. For one the clerk Nikky did not check the product with me nor did I sign my slip and nor did she go through terms and conditions with me. If this is how PEP operates I don't think I would ever go to them. Really disappointed seeing dat I needed the memory card.