If I sell my iPad, is erase/reset enough or do I need to sign-out first?

If you did a factory reset (Settings > General > Reset < Erase All Content and Settings) it asked you for your AppleID password at that point, right?

That should, while factory resetting, dis-associate your AppleID. If you're seeing the "Hello" welcome initial start-up screen, you should be good.

Prior if I recall correctly iOS 9, e.g iOS 7 and 8 you had to sign out, and then wipe all settings as Find my Device was still left active if you don't.

Since iOS 9 and later Reset all Settings signs you out of iCloud and also asks to disable Find my Device, but you will still see the device on icloud.com web or in your iCloud profile. This does not affect device it self, but it can still use amount of paired devices for Apple Store and Apple Music. So if you want fully remove device from your account, which you sell, just delete it from your Devices in icloud.com