Who should file the USPS claim, the seller or the buyer?

What USPS's rule says

“Either the mailer or addressee, for damaged articles or articles with some or all of the contents missing.”

“Either the mailer or addressee who is in possession of the original retail mailing receipt, or in possession of the online label record or computer printout of the Web-based application as described in 3.1d, for lost articles.”

Who holds what USPS asks for

“the addressee must retain the mailing container, including any damaged articles, all packaging, and any contents received.”

“Upon written request by the USPS, the addressee must make this proof available to the local Post Office for inspection ... Failure to do so will result in denial of the claim.”

When a buyer reports damage

1. Ask for photos of the outside of the box, the inside, the packing and the item before anything is thrown away. Damage from shock with no damage to the box isn't payable (DMM 609.4.3), so the outside matters.
2. Ask the buyer to keep everything until the claim is settled; USPS can ask them to bring it to their Post Office.
3. Decide who files. If you file, you hold the label record and proof of value, and the buyer's part is keeping the evidence available.
4. File by day 60 after the mailing date; damage claims can be filed right away (time limits).
5. Handle the buyer's refund under your selling platform's own process; USPS pays the claimant, not automatically whoever is out of pocket.

“Save the original packaging and everything in the package until your claim is settled.”

Worked example: a broken lamp

If both of you file

“If the mailer and the addressee both claim insurance, any payment due is made to the party who submits the claim first and whose claim is approved for payment.”

Two claims for one package don't double the payment. Agree who files before anyone does, so the evidence goes into one claim.

Cases neither of you can claim

• Loss after USPS delivered the package (“Loss, damage, or have missing contents, that occurred after delivery by the USPS.” (DMM 609.4.3)). For Priority Mail Express without a signature, a delivered scan counts as delivery (DMM 503.4.1); for other services, whether a package scanned as delivered really arrived is decided on the claim (what to do).
• A package the addressee or their agent signed for (“Loss after items signed for by the addressee, the addressee’s agent, or delivery employee if authorized under the applicable standards.” (DMM 609.4.3)).
• Scuffs on an item that wasn't wrapped (“Damage by abrasion, scarring, or scraping to articles not properly wrapped for protection.” (DMM 609.4.3)).
• Value that is only sentimental, or replacement value above what the item was worth when mailed (what USPS pays).