Pro Tip for PayPal: in the (hopefully unlikely) event that PayPal sides with the seller, contact the BBB and complain.

I had an issue with a seller on eBay claiming they shipped me an iPad, but it was not to my address. It was considered delivered so both eBay and PayPal decided they wanted to wash their hands of my complaint and not honor their guarantees. This was in spite of the postmaster sending them a letter acknowledging that the package was not delivered to me. Once I filed a complaint with the BBB, PayPal was quick to resolve my issue with a refund.