The act of 'selling out' is mostly a perspective of an individual's sense of integrity or self-worth being 'sold', and thus changing the 'values' of that individual. Three practical examples:
In politics, a corrupt congressman sells out his vote and influence to the highest bidder (usually a powerful lobbyist) rather than to the constituents that voted him into his position in the first place. The politician's integrity is used as a financial instrument, rendering the concept of voting useless.
In the music industry, an artist or band gets signed to a record distribution label to sell out their content to a wider audience, usually for the promise of monetary gain and popularity. Inevitably, the artist or band's content is changed to meet the economic needs of the record label (in layman's terms, the band or artist's musical integrity is changed at the behest of the label's needs--you're creating content for them and no longer you). It's the perception of either 'selling out' or biting the hand that feeds you to keep the original value/principal of your material intact.
In the criminal underworld, selling out is also known as 'snitching'. Some of the most infamous computer hackers have been turned by the FBI to use their skills against other hackers, usually for monetary gain or a lighter prison sentence for their crimes. This opens up a whole new can of worms regarding the moral consequences of entrapment and fed enabling for personal gain, especially in the realm of cyber-crimes.
Although it is mostly seen as a derogatory or 'evil' personal life choice, the process of selling one's self out can also be perceived as an act of survival, though not necessarily for self-preservation in some instances.