Customer
In sales, commerce and economics, a customer (sometimes known as a client, buyer, or purchaser) is the recipient of a good, service, product or an idea - obtained from a seller, vendor, or supplier via a financial transaction or exchange for money or some other valuable consideration.
Professional
A professional is a member of a profession or any person who earns their living from a specified professional activity. The term also describes the standards of education and training that prepare members of the profession with the particular knowledge and skills necessary to perform their specific role within that profession. In addition, most professionals are subject to strict codes of conduct, enshrining rigorous ethical and moral obligations. Professional standards of practice and ethics for a particular field are typically agreed upon and maintained through widely recognized professional associations, such as the IEEE. Some definitions of "professional" limit this term to those professions that serve some important aspect of public interest and the general good of society.
Professional
Scholarship, far from leading inexorably to a profession, may in fact preclude it. For it does not permit you to abandon it.
Walter Benjamin, "The Life of Students" (1915), in Walter Benjamin: Selected Writings – vol. 1: 1913-1926 (Harvard University Press: 1996), p. 38
