When it arrives wrong
Something did arrive, which changes the whole shape of the problem. The claim that wins here is proportionate rather than maximal.
Record before anything else
Photograph the parcel as opened, the contents, and anything that differs from the listing. Do it before handling, weighing, sorting or storing. Evidence produced at the moment of opening carries weight and evidence assembled afterwards invites questions about itself.
Establish what the listing actually promised
Quote it rather than paraphrase it. A great many disputes over the wrong item turn out to be disputes over an ambiguous description, and reading it carefully sometimes ends the matter before it starts. If the listing was genuinely vague, the outcome will reflect that in both directions.
Message before disputing, briefly
Many sellers resolve a shortfall immediately because the alternative costs them more. One message with the photographs and a specific ask is often the whole process. If the release timer is near, dispute first and continue talking afterwards.
Ask for the proportionate outcome
Arbitration weighs proportionality, so a demand ignoring what did arrive reads as opportunistic and tends to get less than a reasonable request would have. Work out the fraction that failed and ask for that.
Damaged in transit against sent damaged
These are different claims and worth distinguishing in your submission. Damage consistent with handling is nobody fault and usually splits. Damage that could only have happened before dispatch is the seller responsibility and is a stronger claim, and the packaging usually indicates which.
Feedback afterwards
Describe what happened and how it was handled. A seller who fixed a mistake properly earns a note saying so, which is more useful to the next buyer than a low rating and more accurate than a high one.
Why proportionate claims win more often
Arbitration is comparing what was promised against what arrived, and a request matching that gap is easy to grant. A request exceeding it forces a judgement about your credibility as well as about the order, and that is a second thing you can lose.
Keep the evidence until it closes
Photographs, packaging and the item stay as they are until the dispute resolves. Tidying up mid case removes the ability to answer a follow up question, and a case that ends with an unanswered question tends to split.