The salesman should try to have a knowledge about the buying policy of his customers. There are some customers who purchase from a store regularly while other do not. There are certain customers, who purchase in small quantities. If the salesman is able to know these policies of customers he can serve customers better.
However these are the general observations. This does not fit into every situation. More detailed observations are necessary for the salesman. Such detailed analysis can be made by classification of customers on the basis of sex, age groups, area, purpose and characteristics.