Here are some useful tips for buying a used piano:
First, look around at several used pianos and check them out for the basics. Try every key with the right-hand pedal depressed to check the tone, and make sure the keyboard, pedals and hammers don’t stick or squeak.
Open the top and use a flashlight to see if all the hammers and strings are in good condition. Hammers must not be string-cut, moth-eaten or badly worn. You should also check for any dirt or rust.
Ask about the previous owner. If it was a professional musician, the piano probably got good care, tuning and frequent maintenance.
Try to find out how old the piano is. Generally, you should buy a used piano that is no more than 20 years old. While older models can be repaired or rebuilt, it is tedious and can be expensive.
When you find an instrument you like, hire a reputable tuner-technician to inspect the piano. He will make sure the “action” (working part of the piano) is in good condition, that the sound board is in good shape and that otherwise the piano is technically in good working condition.
Since a really good used piano can cost upward from a thousand dollars, it is worthwhile to spend time and care in selecting it.