Here are five simple things to look for when buying a used piano from Craigslist:
1. Is the piano in tune or close to it? (If wildly out of tune look out).
2.Do all the keys work well? (Just the fact that they move up and down is not enough, sometimes expensive repairs are indicated).
3. Does the piano have a buzzing sound when played? (Soundboard problems).
4. What is the condition of the case? (Are there any broken legs, is the music desk intact and etc..)
5. Where is the piano located? (Any flights of stairs will cost $$$ to move).
You can find a great deal or a lemon on Craigslist, so please consider the details outlined above. Better yet, narrow your choices down to few pianos and then bring a technician to check them out, it will be worth it in the long run.
For more detailed buying advice you can read this article.