Would it be correct to say "Tarvitsemme myös salmiakkia, koska salmiakkia ei myydä Italiassa"?
Why is it "ei" not "eivät" to translate "they" don't? Is that a generic "one doesn't" in Finnish?
Yes, it's a passive - in Finnish it never has a pronoun and the verb suffix is -(t)aan/-(t)ään. You can indeed imagine that as "one doesn't sell salmiakki in Italy".