Companies like Hearing Aid Buyback or Buy My Hearing Aid specifically purchase pre-owned devices. They typically look for newer models (usually less than 3–5 years old) from major brands like Oticon, Phonak, or Starkey.
Their "SIGHTFirst" program collects used hearing aids to be refurbished and provided to those in need globally.
Premium models with Bluetooth connectivity and rechargeable batteries hold much higher resale value than basic or older models.