What is the ruling on purchasing online educational courses whose value is paid in advance, when not all videos are ready at the time of purchase, and some may have been filmed years ago? Is this considered selling what one does not own or selling with gharar (excessive uncertainty)? Does the buyer incur a sin in doing so, and does the sin cease after the videos are completed?

The contract in question is for a benefit or service, not a sale. Rather, it is a lease for a described benefit that is a liability, for which accelerating, installments, or deferring the rent are permissible. Accelerating the rent is more closely tied to the contract's validity. There is no issue in not knowing whether the lectures are old, new, or mixed, as what matters is their sufficiency for the contracted service.