The verb here, sprzedawać, requires the accusative in positive sentences. So the noun phrase that follows (tasty apples) will be in the accusative, except when the sentence is negative and then it requires the genitive form.
Jabłek is the genitive plural form for jabłko (jabłkach is the locative plural form).
In this sentence, smacznych is the genitive plural form of smaczny.
Correction: "jabłach" is the Locative plural form with one letter missing ;) The word is "jabłkach".
Just wondering, can we not allow "good" instead of "tasty"? I agree that tasty is a much closer translation, but I don't think I've actually heard anyone use it in a while. Most people just say, "good".
I agree with kimasiksin but I would use nice rather than good. Tasty I would use for a cooked dish rather than a fruit. Hae you any suitable alternatives to tasty?
We can add "nice", I guess.
"good" also works (I believe it works everywhere as a synonym of "tasty").