A sellout is someone who claims they wouldn't do something for some type of compensation based on morals or principles and then turns around and does exactly that. A good example would be a die hard rock musician who claims he would never stoop so low as to play country music even if they paid him a million dollars, then he turns around and plays country music because they paid him a million dollars.

The analogy of hating your job but doing it just to get paid is lacking in definition. If I said I would never be a truck driver because I thought the job was beneath me then I took a job as truck driver to get paid then I would be a sell out. If I started in a job and initially liked it but then grew to hate it and find myself still working it to get paid because that's all I know how to do, that's not a sell out. That's life.

There has to be a claimed moral high ground for not doing something and then a surrender of that high ground for compensation which in effect makes you a hypocrite and a sell out.