As a collector and dealer with nearly fifty years of using the USPS and many, many Registered packages coming and going, I can assure you 100% no postal employee would accept or deliver the package shown with the edge cut open(or any of the postmarked area compromised).
As others indicated the Postal Inspector will take the ball and run with it, presumably having a very lengthy(or not
) conversation with said purchaser, who will wish he had never tried this little trick.