This is my understanding, but please correct me someone if it is wrong as I still have trouble with this fun little word "se".
In talking to a Mexican lady at work, to say ". . .donde venden manzanas" would sound like you're saying that the apples are selling something. By putting the "se" in there it sounds like you're saying "where the apples get sold/are sold".
Just my two cents. I'm curious to hear what others say.
