If you are just beginning to look for a good used piano, here's the best guide I know - there's no sense in writing it all out here, so grab your favorite beverage, go the the site below, and start reading toward the bottom of the page. I find this just good old common sense that works...
A Piano Buyer's Guide
Notes From the Piano Bench
Questions and Answers and updated factoids concerning pianos in the 21st Century.