Is it permissible to sell online educational courses by receiving advance payment, then sending the content (videos or links) after a period? And is this considered a delay in taking possession of the sold item? And if it is impermissible, what is the alternative Shariah-compliant method for dealing in this case?

The mentioned method is permissible, for what is being contracted upon is a benefit or service, and it is a lease contract, not a sale. And advancing the fee in a lease is permissible. The Islamic Fiqh Academy has permitted advancing the fee, paying it in installments, or deferring it in the lease of specified benefits for which liability is incurred. In fact, advancing the payment here might even be more conducive to the contract's validity. And the condition of taking possession of both counter-values at the contract session is specific to currency exchange (sarf) and the sale of usurious commodities.