Swappa still exists. Barring that, I don't know.
What is an unused Pixel 10? Stolen? Seems like a red flag. Those are tracked, the IMEI might be banned, and it might only be usable as a WiFi device. It might even be locked out of use, making it a paperweight. If it's not stolen, you basically have a new device, and you probably want close to what you paid for it, but at that point any reasonable consumer would be better advised to just buy it new, and eliminate the possibility of it being locked out.