Purchase pre-owned Yamaha piano, check serial number first for the actual age for this particular piano. If you are in US, then make sure this is not a gray-market piano. Taiwan or China then will not have this problem. more then 20 years Yamaha used piano try not to touch it. If you can find one for less then 20 years then go check the next thing - the sound board. If the back of the piano sound board is very dirty, darkish color, then this board's humid level it high, most likely big chance you will have a crack soundboard, then this piano is useless already.
Most important is, the price need to be in the right range. A used 20/30 years old piano often for sale more then $3000 then its totally not worth. Sorry if there any gray market or used piano dealer ppl out there if you don't like the info . ^_^ In Us market right now, go for new piano is always the best bet since the labor is high for service here compare to China and Taiwan, manufactry warrenty is always good here for 10 years. remember, any STORE warrenty even is 10, 20 or even I can find ppl offer Life time warrenty, those means nothing right now since any piano store can be disappear anytime right now. Just FYI from my 2 cents.