Picture a counter at the end of a busy day with nothing left on it. Everything went out, and whoever arrives now is too late. That plain picture of selling until nothing remains is where the noun starts, and it is good news: a show with no tickets left is a success. The same picture turns hard when what went across the counter was never supposed to be for sale. A promise, a loyalty, a set of beliefs can also be handed over for money, and English uses the same word for that, either for the act or for the person who did it.
From the word’s history
The noun is recorded as a deverbal form of the phrasal verb sell out — sell plus out, that is, to sell until nothing is left.
來源資料註明這個名詞是從動詞片語 sell out 直接變成名詞的：sell（賣）加 out（出去、光了），也就是「賣到一點都不剩」。這條線索正好對上兩邊的用法——票賣光是字面的，把信任賣掉是同一個動作換了對象。
Wiktionary, "sell-out"
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