I guess we should be ready to provide evidence when asked politely, no questions asked, no emotions engaged.
It's a simple cautionary online practice, NOT an accusation to be butthurt about.
And it's probably not wise to force potential buyer to do research about how trustworthy a given online profile is at any given moment - too much work when a simple question can be asked to provide said picture.
It's better to be annoyed by this move (having to ask for such a thing, having to provide the asked evidence) than risking additional layer of scam from buyer perspective. After all it's not a very hard thing to do if you have the item - takes 2 minutes or less and is healthier than taking offense for days.
When online scams are prevalent, I'd argue that people SHOULD ask for evidence as any default trust in things online has been eroded long time ago - this communication will restore this trust somewhat, provided it runs smoothly.
Just a week ago I had to buy 3 items no longer manufactured to have one in good condition despite all deals saying that the item in question is in perfectly good working condition. If I'd been more thorough, I'd spend less money and save my nerves by engaging less in fraudulent/erroneous descriptions.
Trust, but verify. Or as russians say: doverai, no proverai.