According to the metal community, any band that gains the tinyest amount of fame is considered selling out.
So in short, listen to what you like and don't tell anyone else what they think is good music or not.
Dumbass: Omg I hate Dragonforce cuz they sold out and now they are popular and I am dumbass who can't derive my own taste in music lolroflz.
Me: What the shit? You need other people to tell you what sell outs are? That's pretty sad. Just listen to whatever pleases your ears you dumb fuck and shut up about it.