YANBU to be upset but likewise they aren't BU to think it's a scam, you always must take steps to protect yourself by using tracking, receipted delivery etc. It's just a shame you've been stung by such an expensive item.
As a buyer and seller, I've had people try to claim things aren't at described (which is why i take a vid clip the day i send it showing the item from all angles- I've never been asked to produce it the sellers who've tried to be shit have backed off the moment I've responded to say i have it) and I've had things not arrive and people have claimed they've sent, yet not been able to produce anything to say they have. I'd like to give those sellers the benefit of the doubt, I wouldn't berate them over email unless they got arsey with me first or were being unprofessional, but I'd make a note not to buy from again if they were getting funny. I have had one genuinely not turn up and the seller claimed via RM, the item hadn't been delivered or signed for, RM hadn't delivered. The seller was worried I'd leave negative feedback, but I had no reason too, it wasn't her fault it was RM screw up and we had proof, so I left her positive saying shed been professional given what had happened. Had she told me she had sent but had no tracking and then insinuated that perhaps I hadn't realised I'd received, I wouldn't have been impressed and would have thought unprofessional poss scammer.
The negative feedback won't affect much at all and you can 'add an answer to it to explain which people will also see. You know a scammer when loads of their fb is negative, I had one which 100% when i brought but 2 months later it was at 53% due to negative fb-turned out the seller had only sold a bit to get the 100% and then tried scamming.
If you just have one esp with a note from yourself underneath then it won't put people off. Def raise with RM though, it's a shame it's an expensive item.