I’m not an expert, but I though PoP meant nothing in eyes of eBay?
It doesn’t so you’d have to refund the paypal payment. Though it is acceptable for the Royal Mail and you claim back off them for the amount – typically they might ask for the eBay action details as proof of value.
Kaiser- the scams start when the scammer buys something off you (say a nice Hope wheel). He/she complains that it’s knackered (insert any excuse here) so they send it back (tracked). You sign for the box and find a brick inside – eBay see the tracked delivery back to you, not the content so you end up being obliged to refund (well, eBay take it off you..).
Scams where people say they haven’t got something when they have – either send tracked, or wait the three weeks (or whatever RM stipulate) before claiming back off the Post Office.
Proof of postage is free. But will only cover you up to 10 times (I think) value of stamp (so probably £60 these days is it?).