I read a post a while back from one of the EA moderators here that the Global Market is purely based on Supply and demand. The Beach and Mountain flood came as, what I think is, an unintended side affect of the Mayor's contest. Everyone, myself included, is dumping their beach and mountain items to make room for other things.
The only down side of that is that what you get seems to be triggered by what you don't have in way of special items, which is why it's hard to get storage items when you have 28 of each of them, but you're constantly getting more mountain items. Since everyone dumped them in the Market, everyone is getting an abundance of them because of their low supply. Although I have noticed an increase in VU items since the Mayor's contest update, so maybe they do have some flexibility and need to tone back the amount of beach and mountain items they're spawning. I'd rather see more storage and land items.
I'm half tempted to take a few weeks off from the contests and just save money like crazy to buy up all the mountain and beach items till mine are maxed out. It is a prime time to expand those areas to their max.