How can I sell my Apple Watch?

We guys on this forum are users, just like you. This is a user to user only forum. So no, we guys won't buy your watch. But you can take your watch to your closest Apple Store and they'll evaluate the watch and if it's worth anything (not broken, works perfectly, etc.), they'll offer you an Apple Gift Card, which you can use towards future Apple purchases. They won't buy it for cash.

If you want to sell your watch for cash, you could sell it privately.

Depending on the model and condition of your watch, you could get up to $235 by trading it in. https://www.apple.com/shop/trade-in