Does neuroeconomics study the brain or study economics?
As @Kyle beautifully summed-up, 


  Neuroeconomics is the study of the neural basis of economic behavior. 


The terms applies to studies concerning the appeal of new logos for marketing purposes to the understanding of the neural correlate of subjective value. The term neuroeconomics has been subject to some debate which made it sometimes unclear. In particular, the term is ambiguous with regards to its applications. 

Are the theories that are built upon these studies used for marketing purposes (some would say "to optimize brain washing you in the necessity of buying a 7 blade razor") or for the sake of knowing how the brain may be better understood and to the therapeutical applications of these new theories.

My answer would therefore be this: 


the brain is a complex system for which science often used analogies to make progress. It has been compared as steam machines in the 1800s, electricity in the 1900s, or to computers in the this century. 
The complex interactions present in an economic system -as studied by game theory for instance- present a powerful analogy to understand the brain. 
Of course, it thus impacts our knowledge on economics as these are often based on our knowledge of decision making. Thus, your two options are (1) and (2) are interconnected.