You can reset it without WiFi. Using a button combination on startup, you can enter the recovery menu. From there, you can reset the device. Factory reset protection will enable, meaning you'll need to login to your Google account to set it up again.
If you plan on selling it, set it up again, then reset it normally, Factory reset protection will then be off.
How to get into the recovery menu varies by device so look up how to do it for your device on Google or something
