You want to sell your Chromecast but you want to remove any data that may be yours (this may include things such as your Google account credentials). This is sometimes referred as an FDR or Factory Data Reset.
There are two ways as of the time of this post to reset it:
- Hold the button down located next to the micro usb port for 25 seconds until the light starts flashing, then unplug the device.
- In the menu of the Chromecast app there should be a “factory reset Chromecast” option. Choose this and follow any instructions.
More info on Chromecast issues like this can be found at the Chromecast FAQ hosted by Google at: Chromecast FAQ.