More likely, the number was entered when making a purchase at the online Apple Store and requested in-store pickup. In-Store pickup, normally requires a phone number to send messages to let you know when the item is ready for pickup. The number stays there, and is then synced to other places like Settings, but its done over the specific Apple account.
A Previously used computer would have no way of adding the number to the new user's own account at the online Apple Store.
While I can believe Best Buy will sell used computers without properly wiping them, its not really possible for that to affect personal info tied to the Apple ID of the user. The number is not directly on the computer. It gets added to the account on the Apple On-Line store website.