Re-selling a piano requires some research. First, you need to find out basic information about the piano, including brand, size, age and condition. Second, you should get your piano tuned, as an in-tune piano sells faster and for a higher price. Third, research the market to determine an appropriate price.
If you're unsure how to find out basic information about your piano, an experienced piano tuner can be a good resource. Since it's highly advisable to get your piano tuned before attempting to sell it anyway (a tuning receipt provides proof of last service and also includes a list of any additional work that needs done), you might as well ask a few questions of the tuner while he/she is there. There are also professional piano appraisal services that can be used to determine price. An in-person visit usually runs $150-$250, while an online appraisal is around $25.
Once you have determined price (largely based on the brand and condition of the instrument), it's time to put it on the market. You tend to get a higher price if you sell it yourself, so try placing an ad in the newspaper, or on a site like Craigslist or Ebay. You can also search the classifieds yourself to contact potential buyers directly. Other options include contacting a piano technician or piano dealer.
When you have a potential buyer and are ready to show your piano, make sure that it is not only tuned but also clean and free of dust or clutter. Make the surrounding area as clean, comfortable and quiet as possible to give the buyer a positive experience, and be prepared with a list of answers to questions that are likely to come up about the age, condition, brand and history of your piano.