Sometimes payments bounce several days later, usually because of a stolen or maxed-out credit card.
That would be an unfulfilled payment, would it not? As I understand, eBay and PayPal hold all payments for a while to make sure that they do clear.
I've had buyers claim non-receipt even though I have a tracking number showing they received it.
Send it in a way that requires a signature, not just a tracking code.
I've had buyers claim the item was smashed in transit.
Pack your items better.
Seriously, these things are on the seller. Of course there are people out there that are going to try to scam you. They can be buyers, and they can certainly be sellers too. Maybe you just have had a few bad experiences, but to me you sound like one of the many sellers who simply do not care to do business in an acceptable way. Sellers blackmailing buyers with negative feedback was unacceptable and out of control. It was fixed. Your problem certainly exists, but it doesn't matter as much in the grand scheme of things. People will continue selling items regardless of policy. It's the buyers that you don't want to turn away.