Once and for all -

According to the law, Ebay and PP, responsibility of a seller ends with posting the parcel, as there is a third party involved in whose manner of operation seller doesn't have a say or any control, such as APO or courier companies.

Buyer is totally responsible to ensure by whichever means he choses that he will actually receive the goods and seller will follow his instructions and post it accordingly.

The only reason, two reasons actually, that this is not so in practice is the fact that no seller deliberatelly wants to see the buyer out of pocket and possibility of a bad f/back (this applies to Ebay only) 

Buyer who wants the cheapest postage, some even argue about that, they want it cheaper still - and they don't receive the item - have only themselves to blame, complain all they like, threaten with everything, seller doesn't have to give them refund or a replacement.

If they paid with PP, they may or may not get their money back, depends on weather PP believes them, but they (PP) also HAVE to pay the seller back if he complains about unfair decision.