Selling an old piano can be a complex process because many instruments, especially those over 40 years old, can have very little or even "negative" value due to moving and disposal costs. To find a buyer, you should explore several avenues ranging from quick wholesale deals to potentially more lucrative private sales.
Selling to a dealer is often the fastest method. While they offer a lower "wholesale" price to ensure their own profit margin, they handle the logistics of moving the instrument. The best ways to sell or donate a used piano - No Dead Guys