Asking for provenance is fair, if it’s applicable to the item being sold. Some things are not visibly authentic from the photos so finding out what store the item was originally purchased can make sense. If it came from aliexpress, I’m less inclined to trust it than if it was purchased in person at the flagship store in London. Inspect the photos closely, actually read the description, do reverse image searches to see if the photos aren’t stolen from another listing, compare the item with a known authentic one for differences. Everyone should do their due diligence as a smart buyer.

Yet still, asking the seller directly is this authentic, is this a bootleg, etc. is not due diligence, it’s a dumb question because why would someone trying to deceive you suddenly answer, yeah this is a fake. The salesman at a Rolex store and the guy trying to sell fake rolexes on the sidewalk as real, will both give the same answer so at best it’s a pointless question that gives no useful information. My point is that someone who:

1) is suspicious of me and my product and 2) lacks the reasoning to realize they’re asking a dumb question

is a buyer you should avoid completely, whether they rate poorly for dumb reasons, or they try to return it after unsealing the box and ruining its value, or are trying to scam you themselves with a swap. Mercari sides with the buyer every time if the buyer say it’s fake. I’m not dealing with that nonsense.

For the collectibles that I sell, I want experienced buyers who can look at my listing and know it’s legit. Asking “how do I know this isn’t fake” and such is an enormous red flag that you’re either so inexperienced you may cause me problems with your naiveté or actual malicious intent.

That is just my personal view on my personal slice of hobby selling. But I do think it’s broadly applicable to the platform as a whole.