I heard the OOS right before the sale starts theory, too. So far, I was only once able to grab modular buildings during a BOGO 50% sale, but this ways almost two years ago. I remember it was somewhere near the beginning of 2010.
However, this is when the BOGO 50% started at midnight. I just waited around and grabbed them immediately after the sale started. Since then I can never manage to check the site at the beginning of the sale, so I can't really say if the buildings go out of stock right before the sale or not.
Either way, I do hope that I get a chance to put one of these sales to use to get the Pet Shop and Grand Emporium, as other than the TRU sales there just doesn't seem to be any other occasion to get them at a significantly lower price.