I guess the term "sell out" means different things to different people these days...
I always took it to mean someone who compromises their principles or what have you to no end just to make a buck. Like "I'll play/wear/say/do anything you want me to, just please let me play music!" They would have no standards or personal limits at all. YMMV because everyone has different views on what is important or right/wrong, but I can honestly say there would be a "line" to what I would do to make a buck as a musician.
6 figures to back Bieber? Yeah I'd probably do that lol...nobody would know who I am or that I even exist on stage, so what would I have to worry about? I'd just hate having to hear/play the same songs every night for weeks on end, but if the money is good I could deal