We do not have a guide in the works for this specific scenario. It is similar to the user login scenario except that you cannot rely on the userID session variable to filter the recordsets. Due to this difference it would be a custom solution that will be based on the specific requirements that you have.
In order for users to make a purchase and get back to the site at a later time to download you will need to have a user login portion.
Otherwise you can just implement the solution but without any of the filtering for the user, however the other filtering you will need to do will depend on how you need it to function specifically.