One who betrays a cause for personal advancement.
The popular definition of sell out allows it to be used as an insult towards anyone in show business. On the contrary, there are many bands which are considered sellouts which are not.
For instance, Green Day. They are considered sell-outs because they are a punk band who was popular on MTV. Green Day never stated that they would never be on MTV, thus they are not sell-outs in that sense. They never changed their musical style for monetary reasons, thus they are are not sell-outs in the more literal sense of the word. If Fugazi were to sign up with Sony records, they would be selling out because they preach against major labels.
"Man, you are totally a sellout!"