When I see a card that I'm not convinced is authentic and I suspect the eBay seller doesn't really know either, I will ask the seller to take the flashlight on their phone and point it at the back of the card real close. And I ask them to report back to me whether they can see the light through the card or not (I don't tell them what to look for, I just ask them to report to me what they see).
For those of you who don't know, most vintage cardstock will completely block the light. It's not true for every single vintage cardstock, but it is for most. The light typically shines through fakes and reprints. If they report to me that they can see the light through the card, I let them know the card is not authentic. Every seller I have had to tell this bad news too has been thankful, and they have taken the card down from eBay immediately.
Of course an unscrupulous seller on eBay could lie to you, but I have found this approach to be very effective at spotting unwitting fake sales without me being a jerk.
Put me in Coach, I'm ready to play. Today.
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