I've sold a lot of stuff via eBay. I always use signature confirmation. One time, months after a sale, a buyer complained to eBay that he never received the package I sent and he wanted a full refund from me. I keep excellent records.
I presented proof of the USPS signature confirmation that he signed for the package at his address. That stopped the investigation and shut him up. Some buyers are crooks that hope to get something for free while they don't care if they damage a reputation of a seller.
My close bud has 3000 feedbacks, with one asshole giving the negative feedback. You are right, there are dishonest crooks out there. he told me tons of stories about scam, buyers wanting refund for a “hole” when he’s 99.99% sure he sent him the shirt in excellent condition etc.
From what he told me, even if they get a negative feedback for a stupid remark about delivery, it can be erased by the arrogant ‘quality assurance team’ who decide on which negative feedback will stay or go...