Selling per-node access without creating two entities
It really depends on how you want to display the content to your customers. For example, you could show a teaser of the content page via Views. Access to the full content page could be contingent on node access implemented via the Content Access module. You then just have to figure out how to display to the customer which articles they have full access to - for example, a "Purchased articles" View that shows all fully accessible articles using a node access filter.