When shopping for a used piano, make sure your look inside it. Bring a flashlight. Ask the seller if they would remove the panel beneath the keyboard and the panel that includes the music desk (the music holder).
If you are looking at a grand piano, ask the seller to lift the lid and slide the music desk off. In some cases, the seller may offer to remove the fall board which covers and protects the keys so you can look inside with a flashlight.
This will allow you to examine the general condition of most of the internal parts. You don't need to know what all the parts are called or what their function is. You just need to see the general condition.
Do the parts look fairly new, but dusty? If so, this is a good sign.
In most cases, you will find a coating of dust inside most pianos. This can be a very light coating or a very heavy coating of dust. The heavy coating of dust is often found in pianos over 50 years old. Many times, there will be foreign objects and old parts near the pedal area. Some of these parts may have come loose and fallen down over the years. In some cases, the parts up top have been replaced and the piano works fine, anyway. However, if some of the keys don't work properly, the piano will require some repair by a piano technician.
If the parts look rotted, cracked and some are broken, you should call a piano technician to come in and do a piano appraisal and a repair estimate BEFORE you buy the piano. Another alternative is to look around for another piano somewhere else.
Remember, buying a used piano is similar to buying a used car. It's easy for the seller to shine up the outside. It's what's inside that matters more than the outside. Many practice rooms in music schools across the country have beat up pianos that play really well. If the piano plays well (just like if the used car drives well), it may very well be worth buying. The outside is important, but not as important as how the piano plays and was maintained over the years.
Most pianos that have leg, pedal and cabinet damage were, at some point in their lives, moved by the owner. Lifting the piano up is only half the battle. Getting it from point "A" to point "B" without putting a scratch on it is the goal of a piano moving pro. Remember, people can be seriously hurt moving a piano if they are not experienced. I highly recommend hiring professional piano movers to deliver your used piano to your home because it's well worth the money. Plan on about $200 to $600 on the average. Pianos are like cars. Don't get stuck with a lemon!
So when shopping for a used piano, look inside, have a piano tuner-technician appraise and estimate repairs at a later date (if necessary) and hire a professional piano mover.