1) A webpage with salescopy that converts. This is your 24×7 online "salesperson". Once setup, your only activities relate to testing and improving the conversion (sales) through editing the salescopy.
2) A payment link embedded in the salescopy, allowing customers to purchase your product. This payment link would link to a payment gateway like PayPal.
3) A "Thank You" or download page, which customers will be directed to after successfully completing the purchase via your payment gateway.
Once these components have been setup, your website can work on auto-pilot, selling for you 24×7 online.