I believe in the idea that if you really do the hell out of something, you can make up for a lot of shortcomings. Quantity and spirit can substitute for quality in almost all artistic pursuits.

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.”

Sell meant "give" in Old English, including the sense of "give up", "surrender", "betray". (Their word for sell was equivalent to *be-buy.)

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".