I doubt it would ever work in practice, but in theory this kind of thing could be solved by having both parties deposit an additional say 20% of purchase price

I doubt it would ever work in practice, but in theory this kind of thing could be solved by having both parties deposit an additional say 20% of purchase price to an escrow account. So buyer deposits 120% and seller deposits 20%. Once buyer confirms receipt, 120% is sent to seller and 20% back to buyer. If there is a problem, buyer can send the item back to the seller, and once seller confirms (re-)receipt, they each get back their original contributions. However, if buyer claims the item wasn't received and seller claims to have sent it, the money remains in escrow. So the side that's lying doesn't benefit. They actually lose 20%. And therefore there's no rational reason to attempt this scam. However, in the real world there are several problems. The person being "scammed" would be out 120% of the value (whether they're the buyer or the seller), which wouldn't fly with most people, even if the scam is theoretically unlikely. It would also require significantly greater trust of Amazon. Something like this might work with hyper-rational actors using something like a bitcoin multi-sig address, but not for Amazon. Which means I'm back to not seeing any perfect solution to this kind of problem.