Selling out refers to the compromising of one's integrity, morality and principles in exchange for money, 'success' or other personal gain. It is commonly associated with attempts to increase mass appeal or acceptability to mainstream society. A person who does this, as opposed to following the original path s/he laid (or claimed to lay) out for him/herself, is labeled a
sellout and regarded with disgust and immediate loss of respect. Selling out is seen as gaining success at the cost of credibility. Every person, from the entertainment industry, to political revolutionaries, to a Bohemian who suddenly switches to a socially conservative lifestyle, could be a sellout.
http://en.wikipedia.org/wiki/Selling_out