The point of purchase refers to the area where goods are promoted in a retail store while the point of sale refers to the area where the actual transaction takes place. So the point of purchase could be defined also as strategic area for the placement of goods in a store while point of sale is the checkout area where the goods are payed.
Generally these concepts are related with the same thing- a place where the transaction between a merchant and a customer, in a retail store, is made. However there exists a contextual difference in using these terms. Therefore from the viewpoint of customer the area where transaction is made is named "point of sale" while from merchant point of view this area would be named "point of purchase".