Unlike wines, pianos tend to deteriorate with age. The tone in the last stages is soft and dies away quickly. There is no such thing as a piano that is 'good enough to start on'. When Johnny quits his lessons, it may be because it was too hard for him to practice on the old clunker that someone gave you. Many pianos have had hard use, and infrequent service for most of their lives.
you would be wise to enlist an independent piano tuner to assess the piano. A piano technician can appraise an instrument using specialized tools to check the torque of the tuning pins. The minimum recommended torque is 40 inch/lbs. Anything lower may not allow good tuning stability. Replacing tuning pins is a major repair. The tuner should evaluate the piano and give the prospective buyer a list of necessary repairs.
One of the pianos to try and avoid is the “over-damper” pianos. These usually have problems with the damper mechanism and have been found to be a lot of hassle to repair by tuners and technicians.
It may not be wise to purchase a 'drop' action or spinet piano. The repetition of the action is often too slow for all but the simplest pieces. A repair that takes only a few minutes on a full size piano may take over an hour on a spinet.
Be aware that there are a lot of second-hand pianos being imported from Asia and you really need to “proceed with caution” on these as many have been excessively used in schools, etc.
When purchasing from a store or a private piano tuner make sure that they have some type of warranty which include the pin block, frame, soundboard. The person selling these instruments should be able to place a 5 year warranty on the above. Also make sure that the piano is tuned in your home after delivery, this is usually done about 1 week after delivery with a new piano and should also be done with a piano that is second-hand .
unlike new pianos, when it comes to buying a 2nd hand piano more than 30 years old, the brand name will make a HUGE difference in the quality of the instrument and also the purchase price.
look for a water line, if the piano has been flooded, it may be a bad buy.
Play each key several times in a row. If a note fails to sound or will not repeat, the mechanism may need repairs or adjustments. If there is buzzing, the soundboard may be cracked.
Play each note in turn from the bottom to the top. Are there any dramatic changes in pitch or tone colour? If there are, the instrument may need to have strings or hammers replaced.