Sorry, trading in of your Kindle is possible, but not your Kindle ebooks. Once you buy a Kindle book, it is forever yours, unless Kindle revokes the user account. Amazon Trade-In program is for things like textbooks, electronics, video games, DVDs, and more. This product is not generally received by Amazon but a third-party merchant and you would get an Amazon.com Gift Card for the amount your product was sold for.
If in case you want, you can loan your Kindle Book.
For this you have to go to the Product Details page, and one thing is that while you loan your book, you cannot read it, during the 14 days of loan period.