"What does the buyer want to buy?"
Translation:Mnunuzi anataka kununua nini?
They come from two different verbs: "kununua" (to buy) and the origin of mteja cannot be found (in the dictionaries I know/use - nor my father :)) Customers (in different/other contexts) must not necessarily buy something. A spa customer/in a hotel etc. (though they do pay usually. :))