There are several reasons why you might be curious about what to do with an old hearing aid. Perhaps the old aid belonged to you and you have upgraded to a newer model. Or maybe it belonged to a family member that no longer needs it.
Your first option, if the old hearing aid was yours, is to keep it as a backup in case something happens to your new hearing aid or you have to send it off somewhere for service. This is not a bad idea, in the same way that keeping your old pair of glasses as a backup is a good idea when you get a new pair.
You could consider selling the hearing aid on Craig’s List or eBay, especially if it was originally expensive and it is still in good working order. Before you do this, make sure it is legal in your area to resell hearing aids; there are many states in which it is not. To attract qualified potential buyers, make sure that your ad contains all the relevant details including the manufacturer and model, the date of purchase and current condition, warranty and battery details and the reason you are selling it. It is a good idea to include pictures. Look at other ads from the past few months to help you set a price. Do not expect to get back what you paid for it.
However, one option that is available to you if you can afford it is to not sell or keep the hearing aid but to give it away to someone who is in need of one. There are organizations that specialize in collecting used hearing aids and redistributing them to people who need one, but cannot afford them. In the United States, organizations that specialize in the charity collection and redistribution of hearing aids include the HARP Program by Lions Clubs International, Hear Now by the Starkey Foundation, and Sertoma via SHARP. Worldwide Hearing is an international organization that specializes in distributing used hearing aids in developing countries.