A certain piece of art is worth $10,000 for a matched pair. A single is only worth $100. You own an antique shop and have a single in the back. A poor widow with ten children walks in and offers to sell you a single, which you immediately recognize as the match of yours. Are you halachically and/or ethically obligated to offer her more than $100 for it?
I have some thoughts and related questions, but no direct sources.
If you have an infection and your life can be saved with antibiotics worth $5, you don't have to give the pharmacist more than the $5 he asks. If your $10,000 heat pump is broken in deep-freeze weather and $200 can fix it, you don't have to give the repairman more than the $200 he asks. So in our case, you don't have to offer the widow more than $100. You could give her thousands and even marry her if you wish, but you don't have to. The benefit to you personally should not be a factor in how much you pay for something. And it should make no difference whether the seller is our widow or Al Capone.
But in our case, the widow does not know the value of her article. If she asks you "How much is it worth?", then "$100 on the market" would be an honest answer. [Leviticus 25:14] Are you obligated to add that if you have a pair, it's $10,000? Not sure, but probably yes. Are you also obligated to disclose that you have a match in the back? Most probably not. What if she is smart and asks "How much is it worth TO YOU?", what should you say? If you lie, is that putting a stumbling block before the blind, which is forbidden by Torah [Leviticus 19:14]? If you tell the truth, she may ask $5,000, and then you start a bargaining session.
If you pay her only $100 and she later finds out it was worth a lot more to you, can she have a bet din invalidate the sale under הוֹנָאָה (when buyer gives seller less than a sixth under true value of item sold)? (But the site http://businesshalacha.com/en/folder-money-matters/unfair-pricing says that hona-ah does not apply to collectors' items.)