Last year Amazon unveiled the Kindle trade-in program, where you can send Amazon your old Kindle and they will send you a check, gift card or credit you can use to buy a new Kindle. You can assess the value of your Kindle by visiting the trade-in page and if you want to go ahead with sending it to Amazon, they will give you a free shipping label. This program does not get much attention and many people are unaware it even exists.
The Kindle trade-in program is only available in the United States, so if you live in another country you are out of luck unless you use a redirection service. Amazon does not pay a lot for your old Kindle, even their most expensive device, the Kindle Oasis, only nets you $100. You would probably get more money selling it directly on Reddit or Ebay, but you have to go through drama have talking with buyers, sending it out or possibly getting scammed. Amazon seems like a safe bet.
Would you use the trade-in program to sell your old Kindle?