At this stage, I would ignore.
It can be very unsettling to receive messages like this if you've done nothing wrong, and for many people the first instinct is to explain and clear your name, but if the seller is deliberately lying and possibly even falsifying evidence, reasoning with them is not going to work, because they already know they are the ones who are lying and that you didn't do what they accused you of, therefore their goal is something different.
It sounds like scare-tactics to me, if everything you are saying is true, so any response is likely to feed into it. I would have to wonder why they sent photos to you, to be honest. I've had buyers return things that were not what was sent in order to defraud me, and images were certainly taken of the entire opening process, but there was no point in sharing those photos with the buyer. A) That was evidence for who I was reporting the incidents to, and B) the buyer already knew they hadn't returned what I sent them, and presumably already knew that I knew, too.
They can certainly file reports, which might even result in an investigation, if their evidence is compelling enough, but again, if that was their plan, why warn you about it and give you a chance to strategise a defence?
If you feel the need to reply, I would keep it short and simple - something like the monitor was returned in as received condition via Australia Post on X date. Do not contact me again. (Once you advise them not to contact you again, you can then report to eBay if they persist).
