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".