There's a lot of gray to these sales in my eyes. I see a hierarchy of moral calculations here, not just one decision. Whether or not someone "saw it first", or already paid means far less to me than whether or not the deal was done in good faith by both parties. That trumps the other two, for me.
To take the "but he already paid" viewpoint here to the extreme, imagine if some lady has a painting in her attic that she knows nothing about. She lists it for $20 in a Craigslist ad for her upcoming estate sale. Johnny on the spot "painting hunter" instantly recognizes it as an original Van Gogh painting worth ~$5,000,000 less than 3 minutes after she posts the listing. He emails her and says that he's been looking for this painting for a long time because it was painted by someone very dear to him. He sends her $20 via Venmo for it + shipping fees. An hour later, a wealthy art collector happens to show up at her estate sale and sees the painting. His jaw falls to the floor and he says "do you realize what you have?! That's a Van Gogh original painting!" The lady says, "Is that good? Someone just sent me $20 for it about an hour ago. I told them I would try to ship it tomorrow." The appraiser responds, "$20??? It's worth millions! Every fine art collector and museum on earth would love to have this in their collection. I could put you in touch with someone who could help you sell it for what it's worth. You might get upwards of $5 million dollars for it at auction, minus some fees. I would give you $4.5 million for it today though." If you still think that $20 Venmo payment is a "done deal", and that she should honor the sale, then I would suggest that it's *your* moral compass that needs tuning, not the wealthy art collector's or the seller's. I'm not saying this is what occurred here, but it's also not that different. I get that the estate sale was being run by some business selling stuff on behalf of the family and not the family itself, and yes, that's a bit different, but she still has an obligation to get as much as she can on behalf of the family who just lost a loved one. I'd be pissed as hell if I was sitting up in my luxury box seats of heaven stadium, looking down as my wife got fleeced by a chipper who gave her $100 for my set of Binion's WSOP chips after I died (or even $1200).