One tip for finding used pianos for sale is to check craigslist.org. Both music stores and individual owners will frequently post used pianos on Craigslist with asking prices considerably less than they would be if posted elsewhere. When buying a used piano through Craigslist, however, it's important to always have the piano assessed by a professional piano tuner/technician before purchasing to ensure the instrument is in good working order.
Another tip is to purchase a used piano from a company that guarantees its products. The website piano4u.com offers a wide variety of used pianos, with piano prices ranging from under $500 all the way up to $80,000 and more. The company offers nationwide delivery, often for free if quick delivery is not a high priority.
An additional tip when trying to buy a used piano is to work with a trusted piano tuner/technician. Through their regular work, piano technicians frequently know of people in the area who are trying to sell used pianos. Plus, they'll be able to directly assess the condition of the piano to make sure everything is working properly and no major repairs are needed. To find a trusted professional piano tuner or technician, talk to local piano teachers and find out who they trust with their pianos.