"They do not sell tasty apples."
Translation:One nie sprzedają smacznych jabłek.
Why was it 'jabłek' in this sentence but in 'nie lubię starych jabłkach' it was jabłkach? I thought they were both genitive
Jabłkach is locative. You probably got confused by the fact that for adjectives, genitive plural and locative plural look the same.
It should be "nie lubię starych jabłek" as well. Are you sure you remembered that one correctly?