Under cash register, I cant buy anything more I've bought all the buildings and tile/road?

The Summer House is the last 3x3 building that was issued. Except for 3x3 buildings, you can buy duplicates of anything else to design your island. Once in a long while, the developers will add new items to the permanent store. The seasonal items (4th shopping tab on the bottom) change monthly to add more variety. Some seasonal items are recycled from previous sets, but there is always a few new items. Retired items sometimes return in the limited edition decoration shop (5th shopping tab).