Most collectibles in the shop are sold out and have a limited quantity just so there are items that appreciate in value. Think of it like trading cards. Only a few of the really rare ones are made and then they are discontinued, making them increase in value drastically. Thay leave them sold out so members are aware of their existence and become interested. Personally I really like this as it adds a new area of fun to the forums and is great for collectors. The forums has become a game itself. I barely do anything else anymore lol. As for certain items that are not collectibles, im sure the reason why certain mailboxes and things like that are sold out. Maybe because if they didnt restrict them, too many people would purchase them and there would be a lack of TBT circulating the community. Then they would have to find more ways to re introduce them. Or it could simply be the fact that they want certain items to feel exclusive to the user. Kind of like a first come first serve basis.