I've seen a lot of definitions so far that arent really correct. But quit a few of them are right on point. A sell out is a person or person(s) that change to become more successful . It can also mean to conform to suit someone else.
An unpoppular band that someone likes may call that band a sell out when everyone starts liking them. Often it is these people who sold out when they became popular.
People also concider people such as bands, actors, actresses, and other icons ETC sellouts when they become very successful and popular. It seems the only way people can avoid selling out is to not become successful and unloved by people.
People who call others sellouts never think that these people who are accused of selling out to make money came it said business' to become successful in the first place.
The people who often concider people to be sellouts are often jealous assholes who want what they have.
The black eyes peas were concidered sell outs after changing one of their songs from "Lets Get Retarted" to "Lets get it Started" to fit the wants of their record lable and whinney bitches who felt offended.