For those that haven't purchased either the Bay Point Duplex or the Carousel, you can pick up the Bay Point Duplex from the store for 30,000 coins and the Carousel for 25,000. The Duplex offers anywhere from 230-470 population depending on your random upgrades, while the Carousel allows for an additional 700 citizens to live in your town. After this less than 24 hour time limit, the two items will expire from the store and will be unavailable to purchase.
As for those that are already in your town, you don't have to worry about them being removed, as Zynga has only marked new items in the store as limited - they won't be taken from your town. According to Zynga's statement, these sorts of changes will apparently encourage users to "strategize" with the current items available in the game, and will apparently give the developer room to add an "increased range of choices in the future."
Ultimately, I'm not quite sure whether that's an accurate statement, or if Zynga is just trying to lure players into spending massive chunks of coins on items they want to collect before they expire. Either way, we'll stay on top of things as usual, and will let you know if this supposed increased variety of items comes to fruition.
Do you already have the Bay Point Duplex and Carousel in your town? Will you purchase additional items just because they are expiring? Let us know in the comments.