That is what PM is for. I personally don't like the post and order system that is currently being used by some of the lfs. I'm not picking on anyone in particular. I like how the regular stores post there specials and expect you to come in. In my opinion if you want to take orders you should do it quietly by Pm's.
If your just a member wanting to do a group buy, then by all means work it out on the board.
In other words IN MY OPINION if you are considered a store, you should conduct your self as one.