This is an audio discussion site which happens to have a sale forum. This is not a sale site that happens to have an audio discussion forum.
It's thought that, generally speaking, members should have some history and contribute something to the discussion prior to using the sale forum. I understand that not everyone is going to like that, but another way to think about it is that when we have issues in the sale forum, it's pretty much always with a newer member. Therefore this is one way to ensure some small degree of quality control (for lack of a better way of putting it).
The PM limitations are at least to some degree a party to that philosophy (and of course to limit spam, as has already been noted).