Many people looking to buy or sell a seek to appraise its value by means of a so-called "blue book" of pianos. A blue book of pianos is a compilation of prices of new and used pianos, which can be used to give you an estimate of a piano's value.
It is vital to understand that piano values are determined by many factors, and a blue book can by its nature only give you a ballpark figure. Similarly, there are online services that offer piano appraisal given basic information about your piano. Although more accurate than a blue book's general information, it is by no means the whole story when it comes to determining piano value. If you're seriously considering investing a lot of money in a used piano, it's imperative that a qualified piano technician perform a professional piano appraisal in advance. This will cost about as much as a tuning, and it may even end up saving you thousands.
That said, a blue book of pianos is a highly useful starting point, and I'm happy to recommend it here. The online Blue Book of Pianos has a wealth of information about used pianos, and their E-book of Pianos contains price lists for most every brand on the market. The e-book is objective, too, in that it compiles actual selling prices for used pianos of most makes and models, so you can easily see the price range within which your model typically sells.
Keep in mind that the E-book of Pianos is exclusively a (very useful) listing of piano prices. The Blue Book of Pianos website contains an entire book's worth of high-quality, professional and well-researched information at no charge. This includes guides to buying new and used pianos, price comparisons, piano appraisal, selling and moving information, piano construction and tuning, and even an FAQ on digital pianos.
A little caveat: the "real" e-book, which contains used piano prices for pianos manufactured between 1947 and the present, categorized by brand, model and condition, is only available via paid access to the site. As of this writing, that costs a very reasonable $10. However, even though it's called an e-book, it's currently only available online. I am told that a PDF version is in preparation, though, and that will make the guide easier to use. (Strangely, the e-book is not listed for sale at ebookofpianos.com—only at bluebookofpianos.com.)
In summary, the Blue Book has indispensable information for anyone interested in buying or selling a piano. If you're a seller, the price guide (the E-book of Pianos) can help you set a fair price for your instrument. If you're a buyer, the modest $10 investment could potentially save you thousands!