"What are the shipper's responsibilities? I assume they should refund my money and file the claim with USPS.
What are the buyer's responsibilities? I assume I return the broken item. Who pays the S&H on the return?"

It's been my experience that the shipper must file the claim as he/she has the proof of insurance receipt. USPS will send the other party an inquiry, to verify the shippper's claims. Usually the broken item is turned over to the post office as evidence, as ownership of the property will belong to them after they've paid the claim. Sometimes the seller will refund the purchase price immediately to the seller as soon as the claim is filed ... on other occassions, a seller may wait until the claim is paid before settling up with the purchaser.