Yamaha has taken a public stand warning against the purchase of a used Yamaha piano made for the Japanese market. When Yamaha first began exporting pianos to the United States, the company found that some pianos sent to areas of the U.S. with very dry indoor climates, such as parts of the desert Southwest and places that were bitterly cold in the winter, would develop problems in a short period of time: tuning pins would become loose, soundboards and bridges would crack, and glue joints would come apart. To protect against this happening, Yamaha began to season the wood for destination: a low moisture content for pianos bound for the U.S., which has the greatest extremes of dryness; a higher moisture content for Europe; and the highest moisture content for Japan, which is relatively humid. The gray-market pianos, Yamaha says, having been seasoned for the relatively humid Japanese climate, will not stand up to our dryness. The company claims to have received many calls from dissatisfied owners of these pianos, but cannot help them because the warranty, in addition to having expired, is effective only in the country in which the piano was originally sold when new.
My own research has led me to believe that while there is some basis for Yamaha's concerns, their warnings are somewhat exaggerated. There probably is a greater chance, statistically, that these pianos will develop problems in conditions of extreme dryness than will Yamahas seasoned for and sold in the U.S. However, thousands of gray-market pianos have been sold by hundreds of dealers throughout the country, in all types of climates, for many years, and, while there have been problems, particularly in sections of the country with temperature and humidity extremes, I haven't found evidence of anything close to an epidemic. In mild and moderate climates, reported problems are rare. There are, however, some precautions that should be taken.
These pianos are available to dealers in a wide variety of ages and conditions. The better dealers will sell only those in good condition made since about the mid-1980s. In some cases, the dealers or their suppliers will recondition or partially rebuild the pianos before offering them for sale. Make sure to get a warranty that runs for at least five years, as any problems will usually show up within that period if they are going to show up at all. Finally, be sure to use some kind of humidity-control system in situations of unusual dryness. Remember that air-conditioning, as well as heating, can cause indoor dryness.
It's not always possible to determine visually whether a particular instrument was made for the U.S. or the Japanese market, as some original differences may have been altered by the supplier. The dealer may know, and Yamaha has a utility on its website (www.yamaha.com/ussub/pianos/SerialNumberlookup.aspx) that will look up the origin of a particular Yamaha piano by serial number. 
CRACKED SOUNDBOARDS:
MYTH and REALITY
Larry Fine
Solid spruce soundboards swell and shrink with seasonal changes in humidity and, over time, can develop cracks. One of the problems that comes up most frequently in buying a used piano is judging the significance of a cracked soundboard.
Contrary to popular belief, cracks in the soundboard, while often unattractive, are not necessarily important, as long as the tone is acceptable. Very extensive cracking, however, can indicate that the piano has suffered great climatic extremes, and that its life expectancy may be short. In such a case, other symptoms of this will usually be evident elsewhere in the piano. If the cracks have been filled with wooden shims, this means that, at some point, the piano was rebuilt and the cracks repaired.
The ribs run perpendicular to the grain of the soundboard, and therefore perpendicular to any cracks. Any separation of a rib from the soundboard at a crack is a potential source of buzzing noises. A piano with a cracked soundboard should be carefully checked for rib separations before purchase. Repair of rib separations can usually be done at reasonable cost without rebuilding the piano.
When manufactured, the soundboard has built into it a curvature or crown. In a traditionally made, solid spruce soundboard, the crown is maintained by the compression of the wood fibers, whose elasticity causes the crowned soundboard to push back against the downbearing pressure of the strings on the bridges. Together, these two opposing forces enhance the tone of the piano. Over many years, because of the drying out of the wood and the loss of the wood's elasticity, the soundboard loses some or all of its crown, a condition that can be accompanied by the appearance of cracks.
A related condition is that of compression ridges. When a soundboard's compression exceeds the elastic limit of the wood fibers, those fibers may become crushed, producing slightly raised ridges in the soundboard's surface. This can happen, for example, in humid climates, or due to conditions related to the soundboard's manufacture. Compression ridges are quite common, and do not necessarily affect the piano's tone. However, when crushed, wood fibers lose their elastic properties, so the compression ridges are likely to turn into cracks as the soundboard's crown diminishes over time.
Although, in theory, cracks and a loss of crown should result in a deterioration of tonal quality, the actual results vary greatly from piano to piano; therefore, the tone quality of each such instrument must be evaluated on its own merits. In addition, your tolerance for such imperfections will depend on how expensive the piano is, and on your use of and expectations for it.
For more information on this subject, see The Piano Book.