back up MacBook to iCloud

You can't back up a Mac to iCloud, only an iPhone.

You will need to get an external drive and run a Time Machine backup.

However, in saying backup do you mean storing a copy of your iCloud drive documents on iCloud? These are synchronised automatically and are not backups. See Set up iCloud Drive.

So Macbooks dont back up to icloud in the same way that a iPhone uses iCloud backup, to backup everything on your MacBook you need to use Time Machine to an external drive, iCloud Drive will only back up documents and desktop assuming you have it turned on in the iCloud panel in system preferences.

If you are trading into apple you could always buy a hard drive and back it upto this and then if you wanted to keep it you could if not you could always return to apple with their 14 day return policy, or genus bar may transfer the data for you using migration assistant while setting it up in store.