A lot of good ideas. But you have to look at this from all angles.
Your best best is probably to have the package never show delivered. Then you could file a missing claim in a month or so and get the automatic $50 insurance on the package. But if it is scanned delivered with no damage exceptions, then everyone will look at this differently. Shipper will say that either the carrier or the buyer have the knife and shipper is not liable. Buyer will say the shipper never included the knife or the carrier stole it, and they are not liable. The USPS will say the shipper sent an empty box or the recipient took the knife out just to file a claim.
Different mediators handle it differently. Although it did not used to be the case, the credit card companies will find for their cardmember even if the package was scanned delivered. Paypal is just a toss-up since in reality any of the three involved parties could be the crook. eBay will find for the seller. They may reimburse either the buyer or the seller one; but a USPS scanned delivered package will generally protect the seller.
Thus, the main takeaway is that you (sender) can't leave a box in a position to where it can be opened/closed without obvious signs. Then you (recipient) need to have the package delivered to the post office such that you can decline it if damaged and remove yourself from the list of possible crooks. You (recipient) can also meet the carrier on the porch. But if your carrier leaves an obviously damaged package on the porch without a scanned exception - you are in for an argument.
If you want to 100% protect yourself, video the handoff to the USPS including showing them weighing the package. If the buyer, video the clerk handing off the package, including a quick drop on the scales. These are not convenient measures, but otherwise a good crook anywhere in the system can do what they do.
People always have great recommendations, but let me explain why a crook throws a kink in most measures to protect yourself. A smart crook will just surgically remove the contents from the delivered package and then take it to the post office or go straight to a complaint to you. To be honest, they can just keep an assortment of priority boxes laying around and just re-package it if they cause damage. So, unboxing videos are fine - but they don't prove anything. If the seller videos the boxing, so what. If they are a crook they will just print another label and put it on a second (empty) box and send that one. The kicker is the carrier. If he/she is a crook then you don't have a chance. I have seen hundreds of boxes scanned delivered over the years, but the buyer never received the package. A fraction of the time it is on the buyer for not checking the correct place. But most of the time it is a mis-delivery; and a call to the local postmaster with all the detailed information will resolve the issue. The carrier either delivered to a neighbor or wrong apartment box, etc. They are suppose to scan it delivered at the recipients address and the scanner has gps. But I think carriers many times scan it delivered and hide it out for several days to see if there is any ruckus - if so, they can magically find it. I can't prove this, but I have seen packages scanned delivered miles away from the recipient but the carrier comes back and says he accidentally delivered it to a neighboring address when the recipient threatens to contact the regional inspector.
