The best way to find out the value of a used piano is to have it professionally appraised by an experienced piano technician, whose knowledge of local conditions in the piano market, ability to judge the condition of the piano, and memory of recent similar transactions can probably produce a more accurate estimate of the instrument’s fair market value than you can by yourself.
However, if you want to attempt it yourself, you will need to find prices of comparable used pianos that have recently sold. This is difficult to do, both because there is no central registry of used-piano sales, and because to be “comparable,” many factors would have to match, such as age, condition, and location. For the most popular models, it may be possible to estimate the value by looking up prices on Piano Buyer Classifieds, or other places where used pianos are advertised, such as Piano World, eBay, and Craigslist. Keep in mind, though, that these are asking prices, not selling prices, and asking prices are often uninformed and wildly high. However, where many similar pianos are advertised, asking prices tend to settle reasonably close to selling prices.
Alternatively, but less reliably, one can estimate the value of the used piano by depreciating from the price of a comparable new one when the same brand and model are still in production and similar in quality to the used piano in question. First, estimate the street price of the comparable new piano from the Model & Pricing Guide, then multiply by the percentage shown (by age and condition) in the Depreciation Schedule in the article Buying a Used or Restored Piano: How Much Is It Worth? This works best for pianos that are less than 30 years old.
For pianos that don’t fit either of the above scenarios, the generic chart, Prices of Used Pianos, in the article mentioned above, may provide some guidance.
Except as shown in the above-mentioned chart, the value you come up with is an estimate of what a private party would sell the piano for. Prices from dealers are likely to be considerably higher. Pianos that have been rebuilt must be valued differently.
Piano Buyer also offers a fee-based Seller Advisory Service to estimate the value of a used piano, offer advice on how to sell, or how to donate if that is appropriate.