I doubt a tool like that would have much of an impact on the sellers you mention by name; as you note, much of the information is already easy enough to find and those two sellers in particular (although not limited to them) have a strong enough following that I don't think the red traffic light (great idea) would sway them. They keep getting high prices despite the scandals of the past few years, a warning from an eBay tool won't do much.
And in general for cards, particularly graded examples, a limited number of data points on recent sales might not really have much correlation to another card of the same variety at auction, due to the variety of elements at play that make up a card's presentation.
That said, if the potential eBay tool could offer a link to a sales history that is narrowed enough to match the card for sale, I think that would be handy instead of going through the search myself and clicking on all the options. But the tool would have to be reliable enough that I wasn't skimming through a bunch of cards at other grades, for example.
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