Why am I prevented from sending or receiving them ?

Because you haven't made 50 posts.

Because you are Larry Root.

Do the 50 posts count if they are all made in the classifieds section?

Also, once you hit 50 posts, do they automatically turn on or do I have to wait for approval?

I don't think it matters where the posts are made. I've not seen anyone ask how long it takes for approval after hitting 50 so assume automatic.