Acknowledge both sides quickly
Tell the buyer and the seller you have the dispute and when each will hear from you. Silence makes both assume you are against them.
Gather the facts from each side
Ask the same questions of both: what was agreed, what happened and what they want. Ask for evidence such as messages, photos and tracking.
Stay neutral in your wording
Describe what each side says without taking a side. Avoid words that imply blame until you have decided.
Decide by your published policy
Apply your marketplace rules, not your sympathy. If the rules do not cover the case, decide, then fix the rules.
Explain the outcome to both
Send each side the decision and the reason in plain words, and say if there is an appeal. Record the case for next time.