Be very careful buying a used piano. Craigslist has many offered every day and it is one good source, but many times owners think their instruments are worth more than they really are. They may say the piano just needs tuning. A piano needs tuning 2-4 times a year and if it has been let go for a year or more, and many used pianos have not even been tuned in many years, the piano may need a pitch raise, several tunings, or worse, new hammers, strings, or other work. Have a piano technician inspect the potential sale and give you an estimate of the cost of putting the instrument in top playing condition before buying. It may cost about $100 depending on where you live, but it can save a big mistake.
Dealers in new pianos often have many used pianos for sale and usually they are well-tuned and may have some sort of warranty, but even with these, I would consider an independent tech's inspection (not a tech who works for the piano store.) Sometimes prices are negotiable, too. You may pay more than through Craigslist or other sources, but you may have more peace of mind. You can also look at pianos on PianoMart online to get an idea what people are asking. Many of the pianos offered there are from dealers, but some of regular owners. Prices can be accurate or inflated. It varies as much as any other source.