Apple are now offering trade-ins for Apple Watch, at least the Series 2. It's still through BrightStar. If you don't want to follow that link (I'd understand), go to the official Apple Trade-in / Recycling page here and follow the links for trading in a Mac

Below, you can see the trade-in value for a fully operational Apple Watch 2, as of 5th December 2017 (£135)