Depending on the circumstances, interfering in someone else's transaction just might subject you to financial liability. To prevent fraud, probably not. To prevent overpaying - maybe. And it might violate the terms of service of the site where the transactions took place.
Isnt it everyones ambition to buy low and sell high? How do these people do this? Are they misrepresenting the coin? If so, that would irk me.
Once a coin leaves my ownership, what happens with it is none of my business, as long as it is authentic and was properly described by me. Case in point. This would have caused some people an ulcer. I took it philosophically. Behold the 1806 O-129 discovery coin, cherrypicked right off of my Holey Coin Vest. I sold/swapped it for $75-ish at the time (2005). Next guy got $1,801 for it when he sold it on eBay. The two highest competing bidders were prepared to go much higher than that. Neither I nor the person I sold it to (the guy who got $1,801 for it, to his utter delight and astonishment) had any idea it was a previously unknown die marriage. The guy who wrote the article about it (which has since disappeared from the Internet) said: (Mind you, that was before another one had yet been found. Since then, I think one or two others turned up.) Oh, well. Can't win 'em all, right? That one slipped right through my fingers. But I've never been a die variety minutiae kinda guy. I never would have known. The guy who (unknowingly) cherrypicked me had the good grace to send me a copy of the Overton half dollar variety book, signed by him and the person who made the discovery. (That was done as a classy gesture and not as a way of rubbing my nose in it. He had no obligation to me.) I lose no sleep over it. For my 75 bucks, I made a modest profit off of it (since I had gotten it for $40-ish worth of Irish coins and some proof sets). And I have another tale to tell. You have to admit it's a good "one that got away" story.
Good thing I took it philosophically, huh. I felt worse when I had the chance to go detecting at an old plantation site with a friend, called out sick on him when I was feeling a bit under the weather, and then he went out by himself and dug a decent (Fine-ish) 1794 half cent and other interesting things on the site. That scenario had more "might have been" implications than the Overton discovery, since like I said, I would never in a hundred years have known about the Bust half die variety thing. On that plantation site, however, I could have been the one to dig that 1794 half cent, or a Chain cent, or any of a number of other interesting possibilities. At the bare minimum, I missed a grand adventure simply because I was tired and had the sniffles and a sore throat.