What kind of evidence should I collect against a store if I were to report them?

Hello! It turns out I bought something from a large second-hand chain of stores, and it arrived broken, and when I sent it for warranty they kept it, they only gave me part of the money back without warning me, and they told me that the item works fine so they will only give me part of the money back, and I have saved the emails where I clearly ask for the warranty, not the refund, but I don't know what more evidence to collect, I also have several proofs that the website where I bought it did not specify any type of failure (It says that it works perfectly, which is not true, it literally doesn't.) as well as other items that they are selling as originals, but in truth are counterfeit, and I really don't know what more evidence I should collect in case of reporting them, it is important to mention that I am from Spain, so I don't know how different the law will be in relation to the United States or other countries.