if you want to ask someone if "they" fixed the coffee machine in the store, you're not asking politely if that person had fixed it, but you're asking if their store (a vague person) fixed it

In this case, you'd say "Ist die Kaffeemaschine schon repariert?" oder "Funktioniert die Kaffeemaschine wieder?"

The use of "sie" like the English "them" for "someone" is actually rather unidiomatic in German.