In political movements a "sellout" is a person or group claiming to adhere to a one ideology, only to follow these claims up with actions contradicting them, such as a revolutionary group claiming to fight for a particular cause, but failing to continue this upon obtaining power.
In film and television "selling out" refers to compromising the content of produced media, primarily for financial reasons; for example, introducing product placement.
There are two distinct forms of selling out in terms of music. The term 'sell out' referred to those who signed for major labels and those who licenced their music to companies for use in advertising, and more recently it can also refer to those who sacrifice their musical integrity through a change in their musical sound, sometimes due to pressure from major labels
The second definition of selling out refers to putting aside musical quality or original intentions in favor of commercial success, where a distinction is made for those who achieve success without changing their original sound. The difference between the two is often subjective. Whilst artists may change their musical direction for commercial reasons, such as pressure from major labels who require songs to appeal to mass markets a change in sound may also be part of a natural progression of creative maturity.
I wonder how Wikepedia would define a person who takes a job in educational administration or education policy making to help students and then does everything possible to sabotage this education in interest of keeping money in their pockets.