Typically, when I do a factory reset before a trade-in, I let the reset finish and wait for the phone to reboot and show the Setup Wizard screen. I'm pretty sure that as long as you see that show up, your Google account has been properly removed.
It's true that most articles for current phones say that doing a factory reset through the Settings>Backup menu should automatically remove your Google account as well, but I can still find some official support pages that recommend the user manually remove their account first, like this one: [URL="https://forums.androidcentral.com/e?link=https%3A%2F%2Fshop-links.co%2Flink%2F%3Fexclusive%3D1%26publisher_slug%3Dfuture%26article_name%3Dac-vb-p-6631299%26u1%3DUUacUvbUpU6631299%26url%3Dhttps%253A%252F%252Fwww.samsung.com%252Fuk%252Fsupport%252Fmobile-devices%252Fhow-do-i-perform-factory-reset-on-my-mobile-device%252F%2522&token=s7HP3fp-]www.samsung.com/uk/support/mobile-devices/how-do-i-perform-factory-reset-on-my-mobile-device/[/URL]. In general, to be safe, I would still recommend this approach.