Shopping via the Internet
The best way to use the Internet to shop for a used piano is to look for sellers, both commercial and non-commercial, within driving distance of your home. That way, you can more easily try out the piano, develop a face-to-face relationship with the seller, and get a better sense of whether or not you want to do business with them. Craigslist (www.craigslist.org), though not a piano-specific site, seems to have become the preferred classified-ad site for this purpose, as it's both free and is organized by city. If you travel frequently, you should check out sellers in other cities, too—easy to do on Craigslist. Other popular piano classified-ad sites include www.pianoworld.com (which also has extensive forums for exchanging information and getting answers to your questions), www.pianomart.com (smartly organized for easy searching), and www.pianobroker.com. These sites either charge a monthly fee to list or a small commission upon sale, but are free to buyers.
You'll also find pianos for sale on the Internet auction site eBay. Search on a variety of keywords, as each keyword will bring up a different group of pianos for sale. This can be frustrating, as either too broad or too specific a search term may yield unsatisfactory results. The bidding process generally provides a window of time during which you can contact the seller for more information, see the piano, and have it inspected before placing a bid. This is definitely not a good way to buy a piano unless you have the opportunity to first try out the piano and have it inspected. On both eBay and the classified-ad sites mentioned above, many listings that appear to be non-commercial will actually turn out to have been placed by commercial sellers, who may have many more pianos for sale than the one in the ad you answered.