Amazon seller doesn’t respond

Hi all, I recently bought a cordless tool (~300€) from a 3rd party seller and upon opening the package, the product showed signs of use and one battery was almost empty. I messaged the seller, they answered 3 days later asking for proof, I photographed everything and sent to them, well it’s been 11 days since then and no answer. I just found out about the a-to-z guarantee, how does this work exactly? Do I get a refund immediately? It looks like it is similar to a chargeback claim and hurts the seller rep. I don’t really want to get to that point but I see there is no other choice since he is simply ignoring me. I’m willing to keep the product for a partial refund but I don’t know how can I discuss that with the seller. What should I do ?