What kinds and what kind should both be accepted. I'm having to guess which one of those is the accepted answer.
My feeling is that, "What kind of sales clerks are you looking for?" is a perfectly reasonable alternate response, that ought to be accepted.
The problem with your suggestion is that 'keres' means 'looking for', not 'want'. If the Hungarian speaker here wanted to say 'want' or 'would like', etc., there are appropriate Hungarian words he could use. But that is not what he said. He said "I'm looking for ... ."
Consider the following:
1. What type of person are you looking for?
2. What types of person are you looking for?
3. What types of persons are you looking for?
4. What type of persons are you looking for?
In informal English, the word after the "of" is often plural. But in the most elegant formal style (university-level papers, etc.), the noun after the "of" is always singular; it is the word "type" (or "kind", "sort", etc.) that is singular or plural (choices 1 and 2).
In informal English sentence 3 is often used. Note that it has a "double plural": both "types" and "persons".
Sentence 4 is used less often than sentence 1, in my experience.