Earlier quoted context omitted.
That's your job. Sellers are motivated to provide evidence. "I just have it lol" is not going to command much premium over your costs.
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.
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.