"What kind of salespeople are you looking for?"
Translation:Te milyen eladókat keresel?
Adjectives that are right in front of nouns (attributive adjectives) don't take plural endings.
I would say that milyenek there is not an attributive adjectives but rather a predicative one -- one acting more or less like a verb, whose subject in this case is azok a piacok.
So it's not "what kind of these markets?" (a noun phrase) but "(of) what kind ARE these markets?" (predicate + subject).
Predicate adjectives do get plural markers.
Similarly with, say, a magas nők "the tall women" (attributive magas) versus a nők magasak "the women are tall" (predicative magasak).