I think it rather self-righteous and mean spirited of the many of you who are trying to make him feel bad about his good fortune. Ageist too. I see no indication in the OP that the gent was in any way unable to look out for himself. If it had been some fellow in his prime, with an equally nice place, and equally unaware of the value of what he was selling, none of you would have said anything but "good score!"
Gemini had no obligation to educate the man or offer him more for the Linn. It might have been nice of him, but strictly beyond the call of duty. If any of you happened to be in a pawn shop or nick-knack store or flea market and found something you wanted and knew to be a great bargain, you'd buy it right up, offering no more than the asking price. Now if the seller in any of these cases was obviously in bad financial shape, needing the money to make ends meet, then things would be different, but only in the sense in which such people deserve charity, and you ought to give it to them. Similarly, if there were some reason to think the gent was in the grip of some self destructive obsession or was in any systematic way unable to look out for himself and his interests (here I think of the kid stretching his tiny savings to buy his first stereo, but not knowing better than to pay MSRP, or the retailer steering him towards crap gear with a huge markup). But if there was no reason Gemini ought to have just given the guy money (if he had it to spare) regardless of the TT's value or even existence, and no reason why this guy was unable to look out for himself generally or being a fool rather than just uninformed in this instance, there's no reason he should have given him more than asking price.
Besides that, you've got your economics backwards. No one is entitled to FMV of their goods. FMV is a function of what sellers and buyers in fact trade their goods at, not just what knowledgeable experts tend to trade those goods for. Widespread information makes it easier to determine FMV, and tends to normalize the actual prices at which things sell, but it doesn't turn FMV into anyone's right. Nor does it mean that paying less, even way less, than FMV for something is per se taking advantage of anyone, even if they have gray hair.
I've no idea whether Gemini is a stand up fellow who does his part in this world, but if he is, then this transaction is no reason he can't look at himself in the mirror. Indeed, he can even say "good for you, you fortunate man"!
