Chiming in late here,
He is unable - for whatever reason - to provide you with information that would allow you to recover any costs for him via your insurance. Therefore he cannot be made whole. From there the decision becomes how much do you settle on.

- You should be on solid ethical grounds stating that you can pay him back anything that your insurance will give you. In order for you to recover ANYTHING, you need the damaged article back, or a UPS inspection at address of delivery. Standard procedure.
- If you stick to that, you are being fair.
- If he can't or doesn't comply, I would have no sympathy whatsoever.
- The buyer must participate fairly too and provide due diligence IF he wants to be made whole.
- If he can't provide it he does not have a right to full compensation.

From a business standpoint, I agree with your approach of a low offer. But now, he's already given you a black eye w negative feedback.
Game over?

My example:
- One day I got a purchase delivered, with damage to the box that the part couldn't possibly have survived. I kept the delivery guy there while I took photos and called the seller. We agreed I would refuse the shipment, and the shipper returned the package to the seller. The seller got the package and processed the insurance claim on his end. The seller was easily able to honor that whole process and shipped me a new part. This was a fragile $800 part.

Cheers,
- Art