Here's a good article shared from the Dead Kennedys fan page.
Personally, I think the musicians have the most crediblity when it comes to attempting to define punk, but still, there's much interpretation. Nevertheless, the quotes never mention how popular a band is, or how many records they sell. If the early punk artists didn't get people to hear their music, it would've died off like a fad.
But I'd also like to add my own thoughts; I don't consider myself high and mighty enough to classify what's punk and what's not. But as you can see from the comments on that page, many fucktards think they've got this whole 'punk' thing down. I think the real poseurs in the punk scene are those people who think they're a punk encyclopedia and save the fucking day by reminding us what's not punk.
Yeah, I like Green Day and the Offspring among other bands. Anyone's free to have an opinion about them, but the moment you say they're not punk/sold out, I will immediately be alerted to the fact that you're a pseudo-punk hipster.