Yes that's hilarious but you're missing the interesting part. How does the seller provide convincing evidence that withstands scrutiny such that a diligent buyer would be satisfied that it is authentic then.

I would suggest checking some of the high value auctions and seeing what their proof is.

It would be trivial to prove you worked at a company if that's how you were given it, you might even have photos of an event where stuff was given out, emails detailing a "welcome pack", I'm guessing it's these sort of things that are used as proof.