Well, If you blurt out a higher offer in the store interrupting a transaction then you have no common sense AND BOOT YOU TOO, but there are other ways to accomplish this outside the presents, or off the premises of store owner. If you bought polished and freshly cleaned coins guess what? that on you too. It's hard for me to do these hypotheticals because each scenario will be different, I would think. Bottom line I wouldn't t do it, unless someone said you can't get away it and not get caught.... risk vs reward
Would the same logic apply if you were watching a collector cherry pick a valuable coin from an unsuspecting dealer? How about if a collector cherry picked a valuable coin in an auction? Would a third party be obligated to let the auction house know that they were about to let a valuable coin go for much less than what it's worth?
A logic would apply, however, it wouldn't be exactly the "same logic". The major difference in these cases is that the "expert" is offering items at a price, as opposed to an ignorant person being mislead when seeking out the advice of an "expert". One scenario involves predation achieved by a violation of trust (intentionally misleading), the other represents taking advantage of a person that should have known better but did not do their due diligence. That being said, based on the specific situation, I wouldn't feel free to take full-advantage of a dealer/auction house if they had missed something. For instance, if I spotted a $1000 coin in a dealers $5 junk bin, I would make the purchase, make them aware of the mistake, and then offer them something like 50% of the actual value. I would find this to be an ethically acceptable balance between their having made a mistake and my exploitation of that mistake. I wouldn't expect this type of thing from people in general, as the "correct" thing to do is not so clear cut. This is in opposition to the dealer/customer transaction, which does become clear cut at some point IMO.
At a flea market, I saw a guy pushing a walker with a box of coins on it and trying to sell to one of the sellers...I waited till he got done and moved on and then approached him and bought some coins from him on the tailgate of my truck. Very colorful guy whose name I won't mention here, but I've run into him at many coin shows.
Now that's a totally acceptable approach in my book. No specifics on what coins you picked up? Graded? Raw? Anything that will pique our collective interest?
Interesting guy, I told him I just had $40 and I got an assortment of foreign coins including some old Islamic and a 1797 British Penny I still have. He also sold me a Libertad that, when I got it home, JUMPED to a magnet. He also had given me his business card and I called him and the next time I met him, he took it back. A really nice and interesting guy, but count your fingers...