So I was on the other end of this a few months ago....
I bought a watch for my wife on eBay. She wanted a mechanical watch but nothing fancy so I found a perfect "vintage" Tissot from the year she was born on eBay. It arrived packed in tissue paper but sadly didn't work. It just wouldn't tick - the most likely explanation was that it had been shaken hard in the post and old watches don't like that. I contacted the seller and at first he was helpful (after all, not everybody knows how to wind a watch) but then started demanding I take it to a jewellers to prove it didn't work.
I calmed him down and we agreed that I would send it back (I paid the postage since it was only a couple of pounds) and he could examine it as much as he liked but (1) I wasn't paying for an expert inspection and (2) no matter what I wanted a refund.
When it arrived back with him, the crazy started.....I got a series of phone calls in short succession (apparently he can get my number from PayPal - I had no idea). He started off angry and quickly accelerated to shouting abuse at me. It wasn't really clear what he was saying but he was heart attack sort of angry. I of course hung up.
I then got a series of messages via eBay basically alleging that I had opened the watch (!), possibly removed parts or possibly just couldn't reassemble it and that I was a scammer and a crook who could expect a visit from the police very soon. He actually told me that he had spent some time at the police station already. A bit later (presumably when the police said they weren't going to do anything) he started making threats about "knowing where I live" and to "expect a visit".
I didn't take them entirely seriously because (1) according to his profile he is a pensioner (2) he lives > 300 miles away (3) at the time I was living on a gated estate with adequate security with neighbours who may or may not have been extremely charming criminals which meant they had some *serious* cameras that covered >£100K of cars on their drive and coincidentally the approach to my property (4) in my spare time I teach martial arts.
Anyway, I forwarded the threats to eBay, told them the item was broken, got my money back and bought a rather lovely Seiko instead.
BTW, the Tissot was £30. There are lots of people out there who take things way too seriously.