I think the glitter solution was not countered and is a pretty good first-approximation to tamper-evidence. Also taking a photo of internals that the end-user can compare.
This would have to be optional; I imagine most buyers do not have a PGP key (and telling them to generate one is sketchy because they may be buying a new laptop to replace a failing or untrusted one; you would not want the private key falling into the wrong hands or being lost to the buyer forever on a broken harddrive…maybe a throwaway just for the purchase, hut then web of trust is impossible).