I sell lots of high value items via eBay, jewellery as a registered business seller, problems are rare and eBay don't always back the buyer.
Here is what I'd suggest:
Get still shots off your CCTV, you could even try snapping these using your phone. Take a photo of the drop off receipt for the parcel. Take a photo of the parcel purchase details - crucially make sure you get the weight.
Send a message to the buyer saying how sorry you are they are not satisfied with the item received.
Upload the photographic evidence to the buyer that you have of your actions (this can be used as part of a case should things escalate).
Exclaim your shock and ask them just how many toilet rolls it took to fill the parcel and for images and a description of the item received so you can open a case with action fraud - the online fraud service and the courier firm.
Ask for their contact details to pass on should action fraud or the courier company ask the police to follow this up.
Keep super civil and polite.
Next look at their feedback. 90% of the problem customers I've had when I check out their feedback and the feedback they've left for others they have a pattern of being an abusive buyer. eBay take this very seriously and ban bad buyers - they're working hard on this, I've noticed much harder over the last 12-18 months. Use the report buyer feature (its anonymous the buyer wont know you've done it) to raise any suspicious activity you find.
Keep your cool, don't make any acusations you cant back, only make phone or online chat communication with eBay in the morning (ideally between 9 and 10am as you're more likely to get Ireland).
More than once I've had things suddenly turn up. More than once I've noticed that buyers who've messed me around and I report plus add to my blocked list are no longer registered eBay users.