Though it's a year-old thread, the question of "the ethics of cherrypicking" were raised earlier, and here's my take on that.
To my mind, "Cherrypicking" can be divided into two scenarios. And to my mind, one of these scenarios is perfectly ethical, the other is not.
Scenario 1: Person A is selling coins. They are a "professional coin dealer", or have otherwise fully researched the coins to the best of their knowledge and expertise. Person B is a customer of that coin dealer, either well-known to Person A or not. If Person B finds a valuable coin for sale that they, personally, know is worth much more than Person A has listed the coin for, there is no ethical quandary whatsoever - Person B can buy the coin at that listed price without needing to "correct" the seller.
Scenario 2: Person A is selling coins. They are
not a "professional coin dealer", or really have any more skill in coin valuation beyond "look it up in Redbook/Internets". Person B is well-known to Person A as a "coin expert" and has been asked to go through the coins to help them value the coins properly, with the understanding that Person B can make a reasonable offer on anything they find while going through the coins. If Person B finds a valuable coin that they, personally, know is worth quite a lot, perhaps for obscure reasons (eg. it's a rare error/variety not listed in
RedBook), it would absolutely be unethical to offer to buy that coin for considerably less than what Person B knows is the "true market rate" for that coin.
The difference is the expectations placed upon Person B. In scenario 1, there are no expectations. Coins are for sale for $X, Person B might haggle the price but in the end either buys them or doesn't. But in scenario 2, Person B is being relied upon, due to their superior knowledge on the subject, for their honest opinions about the value of the coins. Offering a lowball value at that juncture is, effectively, lying. Person B "agreed to help", and then did the exact opposite of "helping". Depending on the nature of the "agreeement", there might even be a legal issue, in terms of breach of contract.
Now, the unambiguity of Scenario 1 can be muddied somewhat if Person B, rather than keeping the cheaply-bought coin for their own collection, turns around and quick-flips the coin on
ebay for instant profits. This is "muddied" if Person A finds out about this quick flip resale (and given they are a well-connected and knowledgeable dealer in that scenario, this seems likely to occur), Person A will then probably gain a negative opinion about Person B, and probably won't be as friendly to them next time they come in the shop. So in that scenario, if "staying in Person A's good books" is important to person B, then perhaps it would be advisable to attempt to "correct" Person A's pricing at that point. But that's not really a "moral" or "ethical" issue, just a "being nice and keeping friends" issue.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis