I asked the ebay seller what the supposed 'stain' was on this card:
https://www.ebay.com/itm/1967-TOPPS-...UAAOSw6Fhd8RFQ
He got back to me and said it must be the '67' written on the back near the card number (which I hadn't even noticed). I replied that PSA clearly messed up the slab and it should have been labeled as marked, with an MK qualifier, which is generally considered much worse than an ST by collectors, and suggested he revise the listing to let potential bidders know what's going on. In checking back, I was extremely happy to see the seller updated the listing (in the title and the body) to make it perfectly clear what the truth of the matter was. Good on him!!
(No, I don't know the seller or have a horse in this race. It is 100% random.)