The UPS or something can take a photo of the product or item they received. They are not verifying what's inside. They are just verifying the weight, a photo from few angles, available only in case of dispute.
Seller can also do that, but a third party doing it with no horse in the race can act like a notary. We are verifying what we saw. A camera box with X pounds of weight.
If seller wants them to verify the inside contents, he needs to Show them the inside contents. Then UPS will be confirming by photos that yes, an item with this serial number is there inside and we packed it.
Its kind of slippery slope. What if its just a empty camera body? What is its a junk camera? What if buyer says these above falsely?