One of the best ways to navigate through the minefield of used acoustic pianos is to have one you're interested in inspected by an independent, experienced, knowledgeable, and reliable tuner/technician in your area. If it were me, I would hire that piano tech to come out and inspect and even appraise the used piano I was looking to purchase, especially from a private party. If the used piano was in a piano store, then that might be a safer bet, but I may still consider hiring a technician to inspect it.
Not all new digital pianos are created equal. Some are great, some are good, and some are...bad. Most of the time you cannot tell by just looking at them because some look great but play bad. The best thing to do is research new digital pianos before you make your purchase. I can help you with that if you look through my blog reviews on many brands and models of new digital pianos. You can also contact me direct and I will be happy to give you specific advice...at no charge.
For more info on digital & acoustic pianos as well as how to get a new digital piano for less money than internet and local US store prices, contact me at email@example.com or call me direct at 602-571-1864.