get the serial number of the piano and have an authorized yamaha dealer look up the SN in the Yamaha database to find the market it was built for. this is to make sure you avoid gray market pianos for the US market
Well that is a point of view. The opposite point of view is that grey market used Yamaha pianos are absolutely fine and the whole thing is essentially a scare story trumped up by the dealers to reduce competition for the (at one time) very lucrative used import business.
That is a good point and I discussed this topic with the piano technician I had hired. Part of my thought included what you said as the gray market pianos being a scare tactic by yamaha or dealers, but I wondered if there was any truth to it. When asking the tech his response was yes/no. He has worked on gray market pianos and seen some that did have big issues while other gray market pianos were totally fine (regarding yamahas), so he has not really seen anything to prove or disprove it.
I suppose it comes down to what climate you live in. If you live in a really really dry desert climate, perhaps gray market pianos would be a larger issues than somewhere more humid like near the warmer coasts.