Here's Bill Withers on selling out: “Sellout… I’m not crazy about the word. We’re all entrepreneurs. To me, I don’t care if you own a furniture store or whatever – the best sign you can put up is SOLD OUT.”
I think this is the prior that not everyone shares. Yeah if you consider yourself an entrepeneur that has no values except transactional economic perforamnce then its tautological that selling everything is good and the best.
If you however consider yourself an artist, if you think the comercialisation of certain things is inmoral, if you think transactional relationships are hollow or even damaging... then the idea of selling everything as good is nauseating.
Punk in particular is pretty antithetical to the ideas of consumerism and commercialisation. So its a genre and cultural movement where selling out is not only possible, but heavy demonised.
Bill Withers would be juxtaposed to someone like Gil Scott Heron in terms of where their music stands. And he was described as such when he broke out
Will Layman on Scott-Heron said "In the early 1970s, Gil Scott-Heron popped onto the scene as a soul poet with jazz leanings; not just another Bill Withers, but a political voice with a poet's skill."
https://www.etymonline.com/word/sell
Etymonline says the meaning "betray for gain" is from 1200. So this is probably where "sellout" comes from. Compare with "he sold us out".
There's an entry for sellout too: https://www.etymonline.com/search?q=sellout "corrupt bargain".
That makes me wonder if the meaning of a sellout artist was an analogy to an event which became commercially popular, and was (literally) no longer accessible to long-term fans.
I suppose there's often another layer to it, which is that you might think your favorite band (or, say, Apple) has principles and will stick to expressing certain important things. But then they might lose sight of the principles and start churning out lowest common denominator shit for money. It's not as simple then as money=bad. It's more that money as the goal means you have no goal (and your corporate mission statement is a feeble apology for that).