Good chance the USPS wouldn't pay an insurance claim for 1st class mail in this case if you disclose what the package contained. Maybe ebay's free $50 insurance for Priority Mail might cover part of it if that's how you sent it.

Also, you might have a better chance in winning a claim if you paid for signature confirmation. Otherwise, you don't have proof the article actually was received by the purchaser; only that it was put in a mailbox.