There's also the word mnunuzi for "buyer", derived from "to buy" ... but a customer is someone who buys, so it's probably not a terribly important distinction all the time.
i agree with Dlo5. even though the making is the same, the words are different. it is confusing! i think it should be: mnunuzi: buyer (from the verb "kununua" = to buy). and "mteja"= costumer.