I've bought used instruments on eBay and have been happy with them, but those were small, uncomplicated instruments like a flute and guitar which are easy to fix not as big of an investment. You can ask the seller to post a video of them playing the piano so you'll know how it sounds. It will likely need tuned when it arrives though. I might buy the piano, but only after exhausting my search locally first.
Try looking on Craigslist and in your area's Buy, Sale, and Trade Facebook groups. Add your request if you can't find what you need. Also check out pawn shops and get the word out that you're looking for a piano. Check your town's newspaper want ads too to see if a church or family is selling their old one.