"Se" coming before some verbs
 

in the sentence:

Voy al mercado donde se venden manzanas

Why is the se necessary? Does the sentence not make sense by just saying:

Voy al mercado donde venden manzanas

In other words, when do we need to add the "reflexive" pronoun to non conventional reflexive verbs?

Another sentence im having trouble understanding is this:

Es el médico de quien le dije
-What does the "le" mean?

Thanks!!

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.:)

That sounds right to me. There are two issues here:

1) The word order in Spanish is more flexible than English. English usually has subject - verb - object: They - sell - apples. But Spanish often has the subject after the verb, so venden manzanas could mean apples are selling (something)...

2) Spanish avoids the passive voice. Instead of saying apples are sold, you say apples sell themselves. This is where the se comes in. In this case, it means themselves. So apples sell themselves (meaning Apples are sold).

This might sound odd to the English ear. I see signs "house for sale" which say se vende. Obviously a house can't sell itself, somebody has to sell it. But that's how Spanish works.

El "se" en la frase "se venden manzanas" no es reflexivo sinó impersonal, las manzanas están en venta pero no dice quien las vende. No tiene sentido que las manzanas se puedan "vender a sí mismas".
Apples are on sell? sería una buena traducción.

En, el médico de quien le dije, a falta de contexto, puede tener dos interpretaciones: el "de quien le" se refiere a un paciente del médico, que es conocido por ambos; o "le" se refiere a "usted" que está hablando conmigo de ese médico.
The doctor of the person I spoke you about?
The doctor I spoke you about (in a formal speach)?

Both sentences, with or without "se", are OK to me,
= ...where they sell apples.

Voy al mercado donde (ellos) venden manzanas.

Voy al mercado donde se venden manzanas = ...where apples are sold.

"Mercado donde venden" (in quotes) gives a number of results in Google, and the meaning is clear. However, somebody could make a joke in cases like "mercado donde venden animales". :)

"Le" = you.

He is the doctor about whom I told you.

I'd prefer "hablé" instead of "dije" (supossing there is a full stop after "dije"). But there may be regional differences, I don't know.