Just what to do with an old or used hearing aid is a question that arises for lots of people. You could have just replaced your old one with a newer model, you could have received a cochlear implant and thus have no need for yours any more, or it might have belonged to a relative who no longer needs it.
Keep them as backup
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. But first, make sure it is legal to sell used hearing aids in your state. Not all of them allow it. If you decide to try to sell it, be sure to list in your ad all the relevant information a buyer would want to know: the manufacturer and model number, the unit’s age and condition, warranty type, the type of battery it takes, and your reason for selling it. To set a price, look up ads for similar hearing ads, and base your price on the range you find there; it is unlikely you’ll get what you originally paid for the unit.
An excellent alternate option is to give your used hearing aid to someone who needs it. Find an organization that collects used hearing aids, refurbishes them (if needed) and distributes them to new owners based on need. U.S. organizations that collect hearing aids and give them to people who need them include HARP Program by Lions Clubs International, Hear Now by the Starkey Foundation, and Sertoma. Another organization called Worldwide Hearing also collects used hearing aids for redistribution in developing nations where the people need them but cannot afford them.