I tried to sell my Yamaha U3 upright about 12 years ago but the piano shop owner refused to buy. He said the piano is designed or seasoned differently and use different wood as the weather condition is different. I've never heard that until I met him.
Some piano technicians said it is bullshit as Japan is long country , Hokkaido is extremly cold and Fukushima is very humid but Yamaha or Kawai don't design differently for domestic pianos.
Is is true that piano manufacturer design differently for each 52 countries?
I think there is some truth regarding the level of moisture remaining in some of the wood parts used in piano manufacture, particularly the soundboard. That said, there is lots and lots of controversy regarding "grey market" pianos (pianos manufactured in one country to be sold in another).
I do think, however, that some piano dealers use both sides of the argument at times, depending on whether they are buying or selling (no disrespect meant to any piano dealers). If they are selling you a grey-market piano, there is nothing at all wrong with them, and they are great pianos at a great price. If you are trying to sell them a grey market piano, it has a multitude of issues and problems, and they either don't want to buy it at all or offer you pennies on the dollar that you paid for it.
It all boils down to whose argument you want to believe. As for definitive evidence of whether or not grey market pianos are indeed a high risk purchase, I think it is spotty at best. Most all piano dealers sell grey market pianos, usually, and some specialize in grey market pianos. They are out to make a profit, on whatever piano they invest in to sell at a profit.
Also, and this is no dis toward any piano dealers, they know where to buy their inventory at wholesale prices and below, and aim toward selling at retail or close. So, they are not going to buy your piano at a price that is anywhere close to what you paid, although they may offer you a trade-in price on one of their pianos in stock.
I said all that to say this, don't be too disappointed in what the dealer told about your U3 piano 12 years ago. If the shoe were on the other foot, and the dealer had the same piano for sale, it would be one of the finest pianos on earth.